It happens now and then at its own leisure. I tried to describe it in my posts above but then out of the blue it happened so I had to post a picture of it.
The group circumstance I described above can reliable reproduce. It also proves that dcad's selection preference in an ambiguous situation is 'older entity gets selected' and then you can tell the seniority order of surrounding entities by repeatedly pressing enter to see how the selection cycles.
Group in Extruded Solids Test:
1] Create a Plane (any shape) and then duplicate any number of times…
2] Select all the Planes and Group Define...
3] While still selected Extrude (X) the Group so you now have Solids...
4] Gravity-Snap at the extruded end (the end opposite the original Planes) of one of the Solids to select it. Move it somewhere. The whole Solid should be moved.
5] Now Gravity-Snap at the end of a Solid where the original Plane is. Only the Group of Planes will be selected. Click on the Big-Blu handle to Move. Only the Group will be moved. Put them somewhere away from the extrusions. Now snap on one of the extrusions. That Solid, along with it's Plane (that is a part of the Group and now removed from the extrusions) will be selected only. The rest of the Group of Planes will not be selected.
- To explain this:
First, I should explain that an extruded proper Solid consists of three individual entities: a] the original Plane, b] a Grid Surface created by extruding the outline of the original Plane, and c] a copy of the original Plane placed at the opposite end of the Grid Surface... created in that order.
Now, when you snapped at the Original Plane end of the Solid dcad found both the Original Plane and the Grid Surface of the extrusion. The Original Plane is the older entity so dcad gave the order that the Original Plane be selected. The Original Plane has a Group ID and a Solid ID. The programmers decided to give Group selection precedence over Solid selections (since it's more natural that a Solid will be a part a wider Group, than a Group be inside inside a Solid) so all the other entities that have that same Group ID (ie, the rest of the Original Planes) gets selected and the entities that have the same SoliID as the Plane (ie, the Grid Surface and the Other Plane) does not get selected.
Now, when you snapped at the extruded end of one of the Solids the entity dcad decides to be the one selected (the Grid Surface, I think) has a Solid ID, so all other entities with that same Solid ID also gets selected (you guessed it, the Grid Surface and the Other Plane that dcad created from that individual Original Plane AND... that one individual Original Plane... but not the rest of the Group of individual Original Planes, since they don't have the same Solid ID. Each one has it's own Solid ID).
6] Now, to prove, yet again, that dcad selects age over youth: Snap at one of the individual Original Planes (should be still removed from their Solids) to select the Group of Planes and then Delete them (only the Group will be deleted, not any of the rest of the Solids). Now Undo once, to bring the Group of Planes back. Now, when you Undo a deletion, dcad doesn't actually 'resurrect' the exact same entities with the same Entity ID's. Dcad brings back 'clones'. These clones have all the same Group and Solid ID's but not the same Entity ID. The instant you deleted them other entities took their Entity ID's and when you Undo they are assigned new Entity ID's... at the bottom of the list.
With the Group still separated from their Solids turn on Point Select Mode, highlight the points of one of the Group of Planes and move it back to where it came from, connected with it's Solid (with its points exactly on top of its Solid's points). Now press escape to deselect and then snap right back at this Original Plane. This time the individual Solid that you snapped at, including it's Original Plane, will be the only thing selected and nothing else. The remainder of the Group of Original (now cloned) Planes will not be selected. You can not longer select the Group of Planes by snapping at this one Original Plane because it is now a younger entity than it's extrusion entities. To select the Group you must snap at one of the other Original Planes that is still separated from its Solids. If you then highlight the rest of the Group and move them all back to their Solids you would never be able to select the Group again (unless you Undo the move OR, select one Solid and Solid Explode it and move the Grid Surface away from the original Plane so you can snap at it alone).
Now one more thing, more like an observation... If you did put all of the Grope of Planes back with their Solids then Undo the Move. Select the Solid with the single Plane back in place (so only that Solid is selected and none of the rest of the Group) and Group Explode. Now snap at one of the removed Group of Planes. The rest of the Group will be selected but the one replaced Original Plane will not be selected. This is because when you selected the one Solid and Group Exploded, the Group ID of the entire selection was made zero and a Group ID of zero means it is not a part of a group. Since only the one Original Plane was selected and not the rest of the Group only that was given a zero. The rest of the Original Planes retained their Group ID's so they are still a Group. << and this is probably one of the reasons why the original programmers decided to leave gaps in the Group ID list, so if you undo the deletion of a Group and then Undo, the clones can fit right in place without having to figure out what the new Group ID should be>>