Keychain issues on iPad

I find myself in the keychain a series of accounts and related pw, normally created at the time, transformed into very long character strings like MEIEEPgAAAAAAAAAAAAAAAAAAAEwFAYIKoZIhvcNAw****************

The particular feature is that account and relative pw are almost the same, just one character varies (I think) but they are not, of course, the ones I chose. And, just as obviously, they don't work when requested by the sites...

The phenomenon is recent, although I remember noticing something like this looking at the pw in my Mac keychains several macOSs ago - I then thought of some form of coding. But seeing these very long strings being proposed automatically when requested by the site, instead of original accounts and pw, worries me ... does anyone have experience of such a thing?