MacBook Pro 16" M4 - keyboard issue when logging on only. how do I disable the password function to overcome this.

See if this helps

Fix for 789uiojkl keys or numeric keypad … - Apple Community

make certain those keys are working as designed, by looking at them with Keyboard Viewer:

Use the Keyboard Viewer on Mac - Apple Support

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if you bought DIRECT from Apple, not through a Reseller of any description, you have a strict 14-days from receipt for full-value return/exchange for any reason. After that, it's a repair.

I have the exact same issue trying to log into my week old M4 Pro MacBook Pro 16". I don't use the letter O but I do use the number 8 as the 2nd character when logging in.

First character works fine, 2nd character is the # 8 and when I press the 8 key NOTHING happens and I can't get past it. Obviously the fact that I can use the number 8 while describing my issue means the 8 key works just fine.

I have to sometimes reboot 2 or 3 times to get the 8 to input properly and while I'm finishing the login password I get the Mouse Keys flash screen pop up and then disappear. I don't have a wireless mouse paired or enabled but when I query Mouse Keys I can't find a setting that looks like it would disable this irrelevant flash screen.

Very helpful, Thanks Tom.

I found the offending Mouse Keys toggle, per your instructions, in

• System Settings
• Lock Screen
• Accessibility options

Rebooted and login was uneventful