Rec.2020 HLG doesn't appear correctly on ASUS PA32UCXR

I have an FCP project which is Rec.2020 HLG and it looks perfectly fine on my MacBook Pro's 1600nit display. I get the full, glorious, HDR as I should. However, I just purchased a ASUS PA32UCXR external display (also 1600 nits), and FCP & HLG are not working properly on it. The video preview displays something that looks like 50% SDR brightness instead of HDR (although the project's thumbnail appears in HDR).

If I export the movie file and play it back in Quicktime player it shows up correct, in full HDR, so it's not a problem with the monitor or the movie. It's a problem with FCPX, apparently.

If I change the project to Rec.2020 PQ then it *does* display in HDR in the video preview window, so it's something specific about HLG that's barfing, and I don't know what.

I'm wondering if anyone else has this ASUS display and if you've successfully worked with HLG movies on it in FCP.