Error defeating snorkel zombie in plants vs zombies 2

Plants v Zombies 2 error:

Replayed Beach level 10.  Tangle Kelp (level 5) defeats a second snorkel zombie, that submerged snorkel zombie remains, unmoving and unaffected by play, which makes level unwinnable.

First snorkel zombies defeated by tangle kelp resolves normally.

Played game on android phone.