After a little digging, it is actually an unintended side effect of an optimization in the spell-checker that has been in the codebase for a long time. The fix is not completely trivial, but we are looking at what we can do to deliver this in a reasonable timeframe.
In the meantime though, queries that contain any operator in the form <operator>:, like site:, language:, loc:, will not be able to trigger a spelling suggestion.