I had a problem with having CR3 files shown in RT 5.8, too. My files are from a Canon EOS R. I found out that the file extension CR3 has not been in the preferences->file browser->Parsed extensions list; I added it and now I can work with CR3 files, still without correct color management, because I do not have a color profile for my camera.
Maybe CR3 extension must be add to the parsed extensions, in general?
Best regards, Andreas