Hi Phil
You are correct, and i remember wrong... it is when the renderer is set to Software that Prores444 files fail to show... but also only when they have embedded audio.
If you need to use Software mode, i can force it to work by first setting the Project to GPU, import the files, open them in the footage panel, then change back to Software mode and they will decode and show up correctly.
A bit weird workaround, but seems to work.
Another workaround is just to use v16.1, i'm very grateful that Adobe has kept the project file format compatible for the last many releases, so its no problem opening v17 projects in v16.1 :-)
Robert