A workaround for this is to issue fs_restart before attempting to load the model.

hemebond, what do you propose? to do an fs_restart;vid_reload;snd_restart every time the user alt-tabs back, to reload any pk3s or models/textures or sounds that they moved around?...

If you don't like FTE's behaviour, just seta fs_cache 0; cfg_save once.
this will of course make it case-sensitive on linux and harm load times on windows, which should highlight WHY FTE behaves the way it does.
I did try adding file monitoring stuff, but I couldn't get it to behave properly (stupid stuff like no relative paths etc).

hemebond, what do you propose?

The main reason for raising it as an issue is that the inconsistent behaviour causes confusion. FTE could see the model, and prompted me to load it, and then told me it couldn't load it. That made me think the model was broken or in the wrong directory.

I moved the .mdl into several other directories before realising it would load it if I restarted FTE (obviously I know nothing about fs_cache at this point).

In my opinion, if modelviewer won't load it because it's not in the file cache, it should not show it as an option.