I've been making an effort to use . and , , and the same thing happens.
My own feeling is that it's not that the page hasn't loaded, though I have no conclusive evidence. One thing that makes it harder to test is that once hinting has worked on a given page, then it works normally for a while. I think I can confirm that hinting sometimes *does* work even when the page clearly hasn't finished loading - though it's difficult to be quite sure.
Above all, this wasn't happening before. Now it's a frequent irritation, though easily enough remedied by deleting the . or .f and asking for hints again.
Did you recompile the browser to include the searchengine shortcuts? My hunch is that something inadvertently got included that magnifies a pre-existing minor glitch in Vimprobable's hinting algorithm.
It also occured to me that slow hardware may make this much more salient. My hardware is slow, though it's generally not noticeable on Salix.
What makes this really annoying is that (imagining for instance that it really is the page not loading fully) it never resolves of its own accord. You can leave it as long as you like, the only way out is to delete '.' and press 'f' or whatever again, which seems to produce correct hinting immediately.