Letter Avatar Service broken after Update to 2.9.0 beta5

After updating to 2.9.0 beta5 today all Letter Avatars were broken and falling back to the default gray person avatar.

I noticed that the letter avatar service only generates that default image no matter what URL combination I use. Rebuilding the forums again didn’t solve anything.

Forum is running behind Cloudflare in case this matters in any way. Avatars were working fine to the point right before the update.

Any ideas?