The description for the Mixin' It Up advancement is "Place a Mechanical Mixer above the Basin, power it, and start mixing some ingredients.

Whether I do this in creative or survival mode, the advancement is never marked as completed, showing only "2/3" of the tasks have been completed when all three of them have been.

I am in version 1.16.5

This issue has been marked as stale because it has been inactive for 3 weeks. It will be closed if it remains inactive for another 3 weeks.

This issue has been closed since it has been inactive for 3 weeks since it was marked as stale.

I don't know if this is the case, but maybe you need to mix potions in order to get the third task?