Changing macOS admin password. Odd behaviour...

I wanted to change the admin password. I was logged in as standard user, opened System Prefs and into Users & Groups. From there, I changed the admin password.

After (I thought) successfully changing the admin password, I tried using the admin password (in Users & Groups), typing it in (correctly) but it failed. I tried it several times and I am certain I entered it correctly, several times

Oddly, if I type the new password into (eg) a notepad file, then do CMD-C to copy, then CMD-V to paste, that WORKS! ??...

If I logout of my standard user account, try to login to the admin account from a system start (or after logging out of the standard account), neither the old nor the new admin password works. I have to login as standard user, then do the above cut/paste trick where i need admin privileges.

Others have pointed out two things:

1.    Only ever change an admin password from IN the admin account

2.    Stay away from certain characters in a macOS password. My admin password contained an apostrophe, and full stops (periods). I have never read any advice concerning certain characters.

Why might the above behaviour happen?

INFO I am using macOS Monterey, on MacBook Air M1.