I am also experiencing this issue.
Pixel 7 Pro - Stable channel - Receive security preview releases ON
16 (BP4A.251205.006) as of this post (although there's a release pending; notification to reboot is present)
No profiles or spaces or anything in use.
Not using a VPN.
First instance of stuck email was on Nov 22, 2025. Trying to email photos to relatives while at a funeral. :-( I thought the Funeral Home just had bad wifi so didn't notice for several days that the email was stuck.
I am certain that this is a regression, although I can't prove when the last time I sent an email with an attachment from my phone was.
I tried recreating the emails, no change.
I tried rebooting, no change.
I cleared data and cache, no change.
I checked permissions, as far as I can tell gmail has all the access it needs.
Oddly, gchat can send the pictures just fine.
I thought it might be size related, so tried sending gmail's own log file (about 61 Kb), also stuck, no change.
So I also think it's a permissions issue or bug in gmail.
Sending on PC works fine, before and after, so it seems GOS related.
Other relatives using gmail on various Androids are not having this issue, so it's not broadly mobile-related.