IABot seems to be ignoring the local "accessdate" aliases on bs.wiki ("pristupdatum", and "pristup-datum"), which in turn causes it to add another date under a new "accessdate" parameter instead of replacing the existing value in "pristupdatum" / "pristup-datum" – see this diff.
I'm not sure if the bot replaces values in the "date" parameter, but if it does, the local alias for that is "datum".

I couldn't find where to specify the aliases in the IABot Management Interface, and, since it was causing issues, I disabled the bot temporarily.
Hopefully this can be fixed soon so that the bot can be re-enabled.