Standard Htaccess directives disappear by themselves

There must be a plugin that is affecting your htaccess file. (It shouldn’t.)

The htaccess WordPress directives are rewritten when you change the permalinks in Settings. (Which is a good way to get them back if they aren’t there.)

I’d do a text search of all plugin’s code for ‘htaccess’. Once you find the offending plugin, you might consider entering an issue on that plugin’s support forum.