HDR JXL gain map - using base HDR image w/diff methodology vs AVIF?

I'm excited by the addition of better support for the upcoming ISO standard for HDR via gain maps on AVIF and JXL in LrC 14. However, I'm trying to reconcile the resulting files exported (via these instructions) with my understanding of gain maps. I'm also testing them in the linked Gain Map Demo app.

It's clear that "maximize compatibility" means "include a gain map." The increased file size alone shows this—both AVIF and JXL ~double in size when exported this way, which makes sense. However, I'm not sure why the AVIF and JXL are behaving differently in my testing. Everything below is with "maximize compatibility" turned on—I'm testing gain maps, not pure HDR files.

- It appears that the AVIF file is an SDR base plus gain map to get to HDR. I can verify this with how it's displayed in Apple software on various devices, and by comparing the SDR display rendition to what I specify in Lightroom. This is kind of what I'd expect—this is how a traditional JPG+gain map works.

- I'm not sure what's happening with the JXL. "Maximize compatibility" generates a file that still seems to use the HDR rendition as the base image. I.e., on an SDR display, I see whatever software's own tone mapping of the HDR image—I'm not seeing the true SDR rendition.

Is it possible that the JXL is using an "HDR minus gain map" methodology—if that's even included in the standard? If that's the case, I'd have to assume I don't have any other software that can handle rendering this type of gain map.

It's just generally weird to me that AVIF is output as base SDR and JXL is base HDR, though.

So, maybe it's actually a bug and the JXL "maximize compatibility" output isn't actually structuring the file correctly?

I'm just not sure, but something seems odd, and wondering if anyone with more knowledge can shed some light!

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FWIW, the Gain Map demo app sees the AVIF+gain map as SDR-only—no gain map detected, even though it's there. It sees the JXL as HDR-only—whether or not "maximize compatibility" is checked. I realize that app may be out of date for where the standards are at this point, but it verifies what I'm seeing as the "base" image for both, at least.

I'm on MacOS Sequoia 15.0.1, M1 MBP 13" 2020, LrC 14.0. I see this in both P3 and Rec 2020 color profile outputs.