Sounds like a bug. The character might loose the bow due to a status effect (if they no longer meet the requirements), but that should switch them to melee attacks and disable bow skills.
Do you have a save just before this occurs (either consistently, or with a reasonable chance of happening)?
Please email supportdos@larian.com, with a description of the problem and the report.zip file generated by the
D:OS support tool (for the PC version). The report will contain system and game information, as well as your saved games (so whoever is hosting the game should create the report). To reduce the file size, you can delete all but the relevant save(s).
