I'm afraid this is a standard and known difficulty. It affects only a few terms.
The trouble is that pompes funèbres is only ever written in the plural as a compound noun, whereas both pompe and funèbre exist in other contexts in the uninflected singular.
Dictionary content is stored under uninflected terms whenever possible. In this particular case, there are independent entries for pompe andfunèbre (not to mention the ability to link pompe to pomper, etc.). But the term for the funeral parlor only exists in the plural, so it's stored as its own entry under pompes funèbres .
As a result, if you search the WR dictionary for half of a French plural compound noun, chances are excellent that WR will find and return a singular entry for that particular half... and since the singular entry (pompe or funèbre) doesn't actually contain a whole-word-match for any part of the compound noun (pompes funèbres), the code will be unable to locate the entry for the compound noun.
I'm afraid there isn't an easy fix -- nor even a complicated fix -- available for this quirk. French is the main language affected. Bottom line: if you think you might be dealing with a plural compound noun, trying putting the whole thing in rather than searching for individual terms. It never hurts to put short phrase into the dictionary search box... because even if there's no dictionary entry, there may well be relevant forum threads (which will be listed at the bottom).