Freesync not working properly in Parallel core

Specs:
Gigabyte M27Q 170hz Freesync Monitor
8700k@4.7ghz
Geforce 1050 ti
Windows 10
Retroarch 1.9
Parallel core 2.0-rc2 dba9fc8 with Parallel RDP+RSP

I just got a Freesync monitor and the Parallel core doesn't seem to be syncing properly when using Freesync. I've tested this by running games that trigger interlacing, such as Killer Instinct Gold, and using the shader crt-easymode-halation which has interlacing support. I would expect the deinterlacing to flicker at a consistent rate, but this does not happen when using Freesync; instead there are noticeable breaks in the flicker. When using the same setup with Freesync disabled at 60hz (my previous setup), it does flicker at a consistent rate.

I've tried other games such as World Driver Championship, a 30 fps hi-res game, and it does not flicker consistently. There is also noticeable stutter in the text scrolling at the title screen. Switching to 60hz and disabling Freesync solves the issue.

I've tried the Mupen core with Freesync and it appears to work correctly (or at least inconsistent motion is not as noticeable), although it doesn't trigger interlacing in shaders so I can't test that.

Reproduce the issue in the Parallel core with the listed Gigabyte M27Q, GeForce 1050 Ti, Windows 10, RetroArch 1.9, and FreeSync setup. Compare interlacing flicker and 30 fps title-screen scrolling with FreeSync enabled versus fixed 60 Hz, and compare the Mupen core; done means consistent timing without the reported breaks or stutter.