Why does Safari autofill login information with weird username?

That's what Safari autofills in some login forms on different websites:

It looks like a password of kind "Internet password" in Keychain. Safari autofills this information even tho there's a proper "Web form password" saved in my Keychain for each of those websites.

Isn't Safari supposed to only autofill "Web form password" passwords?

Solution 1:

It' an old bug. Update safari to last version. Apple has released three updated versions of its Safari Web browser, which commonly address an issue where saved passwords would not be automatically into relevant password fields as expected. In addition to fixing how passwords are managed Apple issues fix for password autofill bug in Safari | MacIssues

Solution 2:

Have you tried searching for the String/Username "MDoEEPg..." in your Keychain and deleted it?