@Dietmar
Sorry for not getting back to you sooner.
Unfortunately, your delay acpi.sys did not show any change in behaviour for the USB 3.0 on threadripper.
I do still think it's an ACPI issue though - my B550 Steel Legend has the exact same 149C controller in it and it works fine in XP on there. So something in my Threadripper DSDT is causing the issue.
Not to worry though, I've worked around the issue with a separate USB 3.0 card so it doesn't bother me all that much now.