It would appear that if you have more than 16 worms when you level up, it takes you back down to 16 when it replenishes them for levelling up.

It shouldn't do that, did you only have 16 reinforcements when you next entered the game or did it give you the ones it took away from you back?

I still only had 16 when entering the next game...I'll try it again next time I level up and report back now that it's a newer build.

This has been bugged for the team to look into and fix.