I too am having an IOS Mail app issue. For me, I started to notice it in the morning when I'd check email that loaded through the night. The app just freezes. Closing it, closing all open apps, switching to another wifi, switching to 5G and waiting (while the app is frozen) do not resolve it. Only rebooting the phone does and even then, only for 12 hours or so because in the evening, this will repeat and I have to reboot again.
I have 10 email accounts so I disabled all but one and rotated the active one down the list to see if the issue was caused by one of these accounts but the issue was persistent with any one of the ten accounts active and the nine remaining ones inactive. It persisted with all ten inactive. Only rebooting resolves it, temporarily.
Hope they fix this soon.