The version 6.4.2 was released tonight to fix this issue. This update won't join again detached walls, but at least, it will let you join them again and won't provoke this bad bug.
Thanks for reporting it.

Downloaded, installed and tested this morning. [;)]

First: Old projects (<6.4) works fine.
Second : New projects(>6.4) work fine.

At this day, all is good ! [=D>]

Rk: I seem that 6.4.2 is quicker than previous versions. [:)]

Bravo Manu.
.