I did some testing today and found that Time Groups weren't working altogether.
I was using Dynamics 365 v9 trial. I had trouble adding a Time Group to begin with and had to go into God Mode to even add one. Then adding it to a Work Order didn't add nothing to the Schedule Assistant either.
What version of Dynamics were you using? I will be working closely with Microsoft team to find bugs in Field Service. Ill will be bringing this to there attention.