Interesting issue you're up against here. Thanks for bringing this up and to our attention. To be clear, it's specifically the messaging app you're having trouble sending short vids through, yes? If you use Signal or Telegram or another app, do they push through fine through them?
A few people on our team have tested this functionality and have not had issues pushing/receiving short videos over the messaging app on the Ghost Phone / GrapheneOS. Doesn't seem to be an issue with the operating system itself to this extent. Try reading through the following:
https://thedroidguy.com/google-pixel-5-wont-send-mms-1146753
Important to note that sending emojis or video or group texts converts the format of the message from SMS to MMS requiring a data plan. Sounds like the plan you have has a data plan so that might not be the issue, but helps narrow the field of what might be causing the problem. After trying the solutions mentioned (save resetting the whole device, which you don't want to do unless you absolutely have to and have written down all the apps you want to re-install and backedup your pictures/videos should you want/need them) if you're still having the issue can you send a screen shot of the message you are receiving on the device when trying to send?
Hopefully this helps... if not, we'll try and get to the bottom of the issue here!
