Was not able to recreate.
I followed the steps you suggested, created a for...

Was not able to recreate.

I followed the steps you suggested, created a form with a button, in the submit called user_cancel without batch_process(). It works just fine, the user was blocked and I got the following messages on the screen.

The update has been performed.
username has been disabled.

Only difference was I used a static uid, instead of taking it from global $user. Can you try with different uids ?

Cheers,
Swarad