Safari autofill suggestions appear but are not entered into box

Autofill doesn't work in Safari on one of my three Apple computers.

I have searched and searched online for a solution with no luck. Every search seems to lead to the same advice ... go to 'Safari Preferences' ... click the 'AutoFill' tab ... check the 'User names and passwords' box. 

But that can't be the problem because the Autofill settings on all three computers are the same and username and password autofill in Safari work on my iMac and MacBook Pro but not on my Mac Mini M1 (the Mac Mini M1 system has been updated twice and the problem continues in each system which leads me to think that it is a 'settings' problem.).

When I go to a website on my Mac Mini:

---------- My Safari-suggested username (from Keychain Access) for the site appears in a slightly-greyed button correctly. (This is also what happens on my iMac and MacBook Pro.)

---------- However, when I click on the username choice (from Keychain Access) it does not get entered into the Username box. (It gets entered on my iMac and MacBook Pro)

-------------------- Entering my username using the keyboard works. Then for the password box I get the 'From this website' choice indicating that my Keychain is working ... but clicking on my choice doesn't enter it into the password box. I have to enter the password manually to access the site.

In summary, with my Mac Mini M1, Safari successfully gets the username and password from Keychain Access but does not enter them into the boxes when I click on my choice.

I must be missing something obvious ..... help please.