In any of the programs you use look for "MRU" and disable it.
That I believe is the culprit which removes the passwords.
Unfortunately on at least one program I use, I have not found that setting so from time-to-time my passwords also get removed.
Ccleaners MRU setting is in the second block down (Windows Explorer) marked "Other Explorer MRUs"
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If it ain't broke.
Don't fix it.