I have the same issue. To be clear, I'm talking about the Link connection. Who knows what the headset really uses for native apps as there's no way to check that, I think. But checking with the Performance overlay, the refresh rate is stuck at 72 Hz for all PCVR apps running through the link cable, while with version 26 it 90 Hz was working fine. Virtual Desktop still runs fine at 90 Hz. I didn't try anything yet to possibly fix it as I was hoping to find a solution here. Maybe it's not possible to fix it, but then there's a serious bug in version 27, which should be fixed by Oculus as soon as possible.
There is a fix that you can do, by rolling back to v25 but people that have received v28 early said that the issue has been fixed. But if you're impatient, you can rollback the update to v25 from here:
Just make sure to follow the steps on the top comment after download v25 from the post.