here's some more info on my system:
GPU: Intel ARC 770 16GB (Driver Version 31.0.101.5333)
Display: 3840 x 2160 120.000 Hz 10-bit RGB High dynamic range (HDR)
Scaling: 100%, "Let Windows try to fix apps so they're not blurry" is ON
Monitor: LG TV SSCR2
it's a single display setup, no multi-mon, no other software for overlays/etc.
interestingly if I try to use screen recording software the portal does not appear in the recording even though it is visible on my screen - I had to point a camera at my monitor to capture this bug! fwiw regular fences do not exhibit this behavior, it's only folder portals that remain visible at 0% opacity.
there's a driver update available for my GPU I will update my system and report back
edit: updated video driver had no effect. I tested around and it seems that enabling HDR may be a precondition - I wasn't able to see the portal at 0% w/HDR off.
I also tested on Fences v4.22 and it has the same issue