I just thought I'd post about this in case it's affecting someone else. Maybe a google search will find this post and save someone time. I have a Pixel 9 Pro XL running the latest GrapheneOS and I am unable to send MMS from a secondary user profile. It just says "Retrying..." until I switch to the main profile where it thinks the message has been sent but checking with contacts it never arrived.
I spent a few hours diagnosing the issue (Checking my settings, resetting my apps, etc). I searched Google and found nothing.l I finally went on github and searched for a bug and sure enough I found this https://github.com/GrapheneOS/os-issue-tracker/issues/8566
It seems to be a known issue at least and I hope it will be fixed soon.
That's all, have a good day y'all!