I experience hour long delays in refresh for formula-user fields dependent of freshly provisioned users!

To go off what Mark said, when you change an email it will give you a warning that it can take up to an hour for it to cascade down through everything and have formulas recognize the new email. This is likely the exact same thing happening to you when provisioning new users based on what you have said. I think your only choice here may be to wait that hour or less that it takes.

Is there any way you could change the time you are provisioning new users? Set them up a day or few hours ahead of time and then make them active to be picked in your dropdown after time has passed? I think something like that may be the only way to go about it but may not work for you.

I think if you are automating this with a Pipeline it may be possible to just split this into 2 of them. Have one provision the user and have the next run an hour later to assign tasks, something to that effect.