I'm using this build from CurseForge - SpartanUI - 20480ab (2019-10-17). To rule out an older version getting in the way I purged the older builds of SpartanUI from my Dev client. I like using the Classic skin but Player - Target - TargetTarget & I assume Party Frames are all blank and won't take a style. None of the other Styles are working either.

Is this a known issue?

Yes, this is intended at this time. if you are going to run the alpha version of SpartanUI I highly recommend joining the discord and selecting the alpha user role.