There's a strange interaction (read: usually ending up in unwanted behavior) with commands where pathfinding is not the primary task (reclaim, building, capture, loading, unloading, etc). When the pathfinding aborts then the unit 'assumes' all is good and then tries to execute the task it was set out to do. There's no 'retry' for these commands. To look at your case:
Engineer A (left) assisting engineer B (right)
Engineer B is reclaiming, but engineer A was not in range yet so navigates to be in range
Reclaim is gone, engineer B wants to turn back but ends up bumping into engineer A that was still getting in range to reclaim
The 'bump' between the engineers aborts the pathfinding. In this example, the logic of engineer B deems it is ready to build. But the build site is blocked - by engineer B itself! As the logic doesn't 'retry' it ends up being stuck.
This bug you experienced has always been in the game. It just doesn't happen often.
What you can do to alleviate it (in this example) is to use the CTRL + assist feature to not assist the engineer, but instead copy the command queue and apply it to your selection. You can find it in the game options:
It is by far the superior approach to assisting engineers when the complexity of the command queue is high