That is actually a slight overlook we did when programming the swallow skills! Nothing on your part, but something we found after shipping the game and is currently fixing! :)
Since vore skills happen after the prey tries to escape (we're changing it so it happens before), if you use a vore skill which kills the prey as they also escape your belly won't know what to do. Sadly, the only fix you can do is to load your last save and try again and hope good ol' rng is on your side! Don't worry, next patch I make will have it fixed... hopefully XD
Either way, I hope you've enjoyed the game and if you encounter anything else, please share it and we'll fix it as soon as we can!