A cave fisher from Volo's Guide to Monsters has the following trait:
Flammable Blood. If the cave fisher drops to half its hit points or fewer, it gains vulnerability to fire damage.
The trait makes it seems like the first time you reduce the cave fisher to half hit points or less, it is vulnerable to fire damage for the rest of the fight. But that could make some interpretations by RAW differ widely from what was (probably) intended, as finishing a long rest and healing to full hit points would not prevent it from being vulnerable to fire damage. Receiving healing, such as from Cure Wounds or a regeneration trait a cave fisher may gain from the Mournland (just an example-a table for Mournland monsters in E:RFTLW suggests adding regeneration to a monster) would also leave it vulnerable.
So, would a cave fisher reduced down to 1/2 hp and healed back up retain fire damage vulnerability, or would it be removed when they healed?