Partially fixed.
I'm not entirely sure what's causing it, but sometimes it still triggers "regen under fire" event
However, it also tries to use normal shield regeneration at the same time. Your shield charges every second and you'll see that the 1st second it charges your shield with the "regen under fire" modifier applied and the 2nd second you see that it uses the normal regeneration, including the missing part of the 1st second. This repeats itself till the "regen under fire" event is gone.