Writing an email response leave page it disappears

So FRUSTRATING! It happens often I’ll be in the middle of writing a long response and if I leave that page for any reason most of the time when I come back the whole email is gone and then I have to start all over and I HATE it are there any solutions to this??? and why is it happening??? I have an iPhone 11 Pro and it’s pretty full so I don’t know if that’s the problem or some thing else?!?

I’ve been getting in the habit of copying it just as a precaution! Any suggestions would be appreciated:)