Guys, fancy some more data?
Unsure of all the possible conditions, but seems to me that it can be recreated for any weapon and it will happen for all items of the same type acquired to the base directory during the same session (so no save + reload in between). Tested via buying from Cuda.
For example: bought one Abrams and upgraded its damage stat by one point. Then using the shop cheat refreshed Cuda's offering and bought another Abrams. Now the new one has this damage stat upgrade too while its tooltip still shows the old pre-upgrade damage number. Next Abrams has it too, etc. And if I would then upgrade clip size in one of them it will also tic this upgrade in all the others...
However, doesn't seem to influence Abrams that were already in the directory at the start of the play, in other words the game 'links' only items acquired within the current session. Moreover, if you save the game with bugged weapons, after loading it all their tooltips are now updated to contain the upgrade (even higher clip sizes), but haven't tested whether holds in a battle. Nomad, was thinking, is it possible that those first three posts happened to you within one play session?