Trouble exporting HDR video from FCP for YouTube or Plex

What version of MacOS are you running? There may be a Sequoia HDR issue if playing Rec.2020 HLG content in FCP 11 over HDMI to an external monitor. Are you using an HDMI monitor? It does not happen if using a Thunderbolt display, Apple Studio display, or the built-in XDR display on late-model Apple Silicon MacBook Pros.

I just tested it via a USB-C connection from my M1 Ultra Mac Studio to my 3rd-gen 11" iPad Pro, and it seems to also affect that IF the HLG content is played within FCP and using the iPad Pro as the viewer. It does not happen if you play the HLG content in Quicktime on the Mac using the iPad Pro as the viewer.

For Quicktime or Plex playback, what type of monitor is on your Mac and what is the color profile in use, as shown in System Settings>Displays?

You showed three playback images. Are those all being played back from the Mac, using the iPad Pro as the monitor? If one is the iPad Pro, and the other two are some other non-HDR monitor, that will not look the same. If there is a difference between Quicktime vs Plex playback to that monitor, Quicktime is a color-managed app that supports ColorSync. If Plex is not color-managed, it will not look the same. That is similar to a video not looking the same if played back in Quicktime vs VLC.

I have both an Apple Studio and LG 5k Thunderbolt displays connected to my M1 Ultra Mac Studio running Sequoia 15.1 and FCP 11.0. I use various SDR ICC color profiles on the LG, but usually Rec.709 100 nit gamma 2.4. If an iPhone Rec.2020 HLG Dolby Vision video is played in Quicktime Player, it looks OK on the LG SDR monitor. It obviously does not display as HDR, but the color and gamma is as expected for SDR. If played on a USB-C connected 3rd-gen iPad Pro it looks more like HDR. If played on the Apple Studio display using the MacOS P3-600 nit preset, it looks more like HDR.

The same is true when I play the iPhone HLG video in the FCP viewer. On HDR-capable monitors it looks like HDR. On SDR-capable monitors it looks perfectly OK, just not HDR.

That is using Quicktime or FCP playback. If using VLC playback it looks like SDR even on an HDR-capable display. That is because VLC is not color-managed, does not read the NCLC tags and doesn't know it is HDR material.

If I upload that to Youtube, it depends on what browser is used for playback. Currently MacOS Safari does not support HDR on Youtube but it does on Vimeo. MacOS Firefox does support HDR on Youtube. This can be seen by right-clicking on the Youtube playback window and selecting "Stats for Nerds." On Safari it shows 'color: bt709/bt709. On Firefox it shows (HLG)/bt2020.

You can also do that test using existing Youtube HDR/HLG content by querying for "HDR Channel" and viewing one of those. But your uploaded HLG material should behave similarly: IOW, it will look good on a late-generation iPad Pro or MacBook Pro with a built-in XDR display, will not look like HDR if using MacOS Safari with an HDR-capable monitor, but will look like HDR if using MacOS Firefox with an HDR-capable display. If used on a Mac with an SDR display, the streamed HLG material should OK, only like SDR.

When a MacOS app plays a video, if it's color-managed, it will behave like Quicktime Player. This assumes the monitor display profile shown in MacOS>Displays is appropriate for the material. If it is not color-managed and not HDR-aware, it is unpredictable.

Edit/add: However, there is a possible issue on Sequoia 15.1 and FCP 10.8.1 and 11.0 involving HDR content from the FCP viewer (not Quicktime) over HDMI, whereby it doesn't display correctly. I am still investigating that.

“as it's not widely supported yet”

except by every recent iPhone on the planet.