fyi: tech support quickly discovered this was "user error" by a newbie (me) that at some point (probably during the learning process) had turned off angle snaps. Thus, many wall angles were at not at the mathematically precise angle required for a perfect joints between the walls in question. For example, one wall angle was 89.951878 instead of 90.0

As with most very complicated software, it usually boils down to "user error".

Thanks everyone