The website .htaccess file was changed to a different PHP version and there is no indication why because PHP Manager shows the correct PHP version in both WHM and the user's cPanel account.
The issue typically occurs because there are other domains added or subdomains added with the same documentroot but with different PHP versions.
Here is an example. Let's say the user is cpuser and the domain is cpuser.com with a subdomain of whathappened.cpuser.com. Both whathappened.cpuser.com and cpuser.com are set to DocumentRoot /home/cpuser/public_html but whathappened.cpuser.com is set to PHP 5.6 while cpuser.com is set to PHP 7.4. An EasyAPache update can potentially set /home/cpuser/public_html/.htaccess to a PHP version of 5.6 instead of PHP 7.4 using the AddHandler directive.
grep -E '^document|^phpver' /var/cpanel/userdata/cpuser/*
This is easily fixed by correcting the subdomain PHP version in the above example to the same PHP version of anything else sharing the same documentroot.