Tuta and Proton, which are both commonly recommended, have not released their back-end code (read about it here and here). Proton doesn’t seem interested in open sourcing their code, and Tuta has claimed they’re aiming to do it in the future (the claim was made years ago and nothing has happened yet). They also both claim to be open source in their marketing.
Is there any alternative that is fully FOSS?
What does that even mean? Fastmail is mostly or entirely FOSS on the mail backend but the web front end is currently non-FOSS. You can of course use a FOSS client instead.
Hosted services are more about hosting than about the backend software anyway. You can use something like mailcow to self-host email if you don’t mind headaches.
mxroute.com is probably all FOSS but they don’t make an issue of it. Actually idk what web control panel they use now. It was formerly cpanel but they switched, probably for cost reasons. The webmail ui’s are roundcube and a few others.