Shorten the time-to-instadeath of digestion attacks significantly, in order to make monsters that have those attacks a significant threat to the player and not a non-threat where the player can weather the HP damage and attack the monster’s stomach repeatedly until it dies.

Alternatively, remove it as an instadeath on the principle that the instadeath does not make for fun gameplay, and have the “timer” be HP, with a large amount of acid damage dealt each turn. The flaw with this is that acid resistance would nullify it, so if you’re not afraid of a couple turns’ worth of acid damage you can just carry an alchemy smock and equip it. It would also mean that, logically, digesters should not be able to instakill anything that’s acid resistant.