I recently started running UB12 on a W7 Pro laptop, after using UB 2001 and XP (!) for many years. I'm working with a project where the bottom copper layer is a ground plane. Most (but not all) of the components are SMT on the top copper layer, and the ground plane connection is through a short trace and a via. When I place a part and connect it, all is well (i.e.. the thermal relief is created at the via on the correct layer), but I started noticing that while editing in other areas of the board, often I would notice that ALL of my thermal reliefs had disappeared, both SMT and thru-hole, and the display showed a rat's nest of all the ground traces. Refreshing the view did not help. I found out accidentally that if I brought up the netlist editor, and did NOTHING but select the ground net (to which the bottom copper layer was connected) then clicked "OK", all my thermal reliefs re-appeared. This behavior is obviously annoying (although if I accidentally forgot while doing Gerber export, it could be devastating!) and I'd like to know if there is a setting or something that would prevent this behavior. Anticipating the question, I am not sure what specific function preceded this behavior, as I typically was moving components around, editing traces, etc. and often did not notice the missing thermal reliefs until some time had gone by. I just tried to do some editing and force the situation, and was unable to find the "magic" combination.
