Same iOS version here, and "select all" can be done using Apple mail app, as usual.
Try a forced restart of your device, using these instructions:
Press and quickly release the volume up button, press and quickly release the volume down button, then press and hold the side (power) button. Only when the Apple logo appears (it might take 10-20 seconds), release that button (ignore all other screens that might appear, the one with the "slider" to turn the device off included).
Then see if you're using the correct gestures, as per this support page:--> Select, cut, copy, and paste text on iPhone
If the issue still persists, I suggest that you contact Apple Support directly, using this link:--> Contact Apple for support and service , or make an appointment (*) and have your device inspected at an Apple Store or AASP (Apple Authorized Service Provider) among this list:--> https://locate.apple.com/choose-country-region . After checking it out, they'll give you all the available options.
Regards
Giulio
(*) You can do it: