This was a known issue in 3.034 to 3.306. Was fixed in 3.037 and up.
aaaw that would be intersting! j/k.
Btw, this might be an issue, but the game that we encountered this was long ago (in patch-wise). Against tele turtle and shield ninny, on 5 planets:
for 3, we destroyed everything else than orbital command station and warp gate, and build turrets etc to keep the planets intact (calling them A from now on) and many more in the same shape but by dysons
the other 2 planets: we destroyed everything else than warp gate, and occupied those (B)
A planets kept reinforcing, B planets never got any reinforcements. However: we also saw something else that was weird: none of the planets ever got reinforcements. Seriously. So, we suspected that, AIs were trying to reinforce the planets classified as A. That game was fairly easy.
Also, one of those "A" planets were getting Mark IV reinforcements, while our AI process never exceeded II.
We captured all of those planets, and THEN they started to reinforce theirs. That was late anyway, we were bordering their homes already then.