@JLTastet, do you have the raw file and the XMP sidecar file as well, maybe? I’m trying to find a workflow that would be the most convenient in DT with the plugin, and a complete image set would help me.
I made a total overhaul of the plugin (not yet comitted upstream), moving it out of the export UI to a separate UI component, and fixing several issues:

Alas, I’m facing a major block now. DT Lua APIs don’t allow me to modify export’s “global options”, including the color profile. In fact, it’s currently impossible for Lua plugins to export two images (SDR and HDR, in our case) with different color profiles, which your workflow depends on.
I can workaround that by supporting a two step approach, in which exporting some or all of the source files is left to the user, and the exports are then reimported into DT for the plugin only to merge them into UltraHDR (and correct the EXIFs?). Or I could try to figure out how to generate the exports outside of DT. Any preferences?