If someone is interested, why the entire "price bug" thing has happened, I've found the answer on Wadjeteye's discord, provided by Dave Gilbert:
https://discord.com/channels/461404640010043393/753319353147064331/846740285405003786

Quoting:
I had to do the conversion for GOG manually as opposed to every other site which does it automatically so naturally I mess it up. Dang it. Well, early customers get lucky.

Maybe GOG should take a good look at such quotes - especially the part in which he says that EVERY other store does it better. Maybe ... just maybe ... their update system also is not the best and could or maybe even should be improved since similar complaints could be heard in the past about the update process on GOG from some devs as well. If those devs are in fact incorrect and everything is as easy as everywhere else they might instead improve their communication with the devs instead... just an idea.

Thank you for putting up with GOG. I purchased the Wormwood Bundle here today.

Too bad the achievements for this marvelous game won't work on my end(s) here.
Already got like 4 of 'em and... nothing. Just once it worked.