Considering the timing (January 24th), it's probably related to Apple's switch from a SHA-1 signing certificate to a new SHA-256 one: https://developer.apple.com/documentation/technotes/tn3138-h...

It's possible their receipt validation code doesn't support SHA-256, and they can't/won't update it. Maybe the source code was lost, the team was disbanded, or they're just not bringing in enough iOS IAP revenue to justify the engineering cost.