Just wait
There's been no statement that they plan to fix it, and a lot of games do move from itch to other platforms (mostly Steam) and force users onto them. I make a point of only buying games which are playable through itch, and not until I've made sure the devs plan to maintain support for the itch.io version - which seemed safe with Among Us because it's been running fine on here for years already.
Contact support about the issue, the more people that do, the more likely they are to actually treat it as a problem. I'm one of a number of buyers on itch.io who don't have the option of using Steam (Valve has known their client is a health risk for some users for over 18 months and refuse to address the problem).
itch.io support page says that for Among Us, you should contact the devs at us@innersloth.com but recent messages from Innersloth have said to use support@innersloth.com - I contacted the first email address 5 days ago and haven't seen any reply beyond an automated "message received" notification. I sent an email to the address Innersloth have provided yesterday.
Steam support won't do anything for you regarding a purchase outside of their platform and would, like itch does, direct you to contact the devs first.
I'm going to hope I hear from the devs soon, but if not, I'll be contacting itch.io support directly and getting a refund (since I don't have access to Steam for medical reasons).