The issue here is perhaps less black-and-white than the red squiggly lines under underfill in Microsoft Word (and other word processing programs) might lead one to believe.
Because under- is a prefix, it can legitimately attach to various root words to form meaningful words, even if those words aren't acknowledged by most dictionaries. Here is the entry for under- in The American Heritage Dictionary of the English Language, fifth edition (2010):
under- pref. 1. beneath or below in position: underground. 2. Inferior or subordinate in rank or importance: undersecretary. 3. Less in degree, rate, or quantity than normal or proper: undersized.
Definition 3 of under- is clearly applicable to underfill, just as it is to underachieve, underbid, underbuy, undercharge, undercount, underdo, underdress, underemphasize, underestimate, underexpose, underfeed, and underfund—to name just the under- verbs with root words starting with a, b, c, d, e, or f that AHDEL provides entries for. This dictionary doesn't include an entry for underfill—but that doesn't mean that it views underfill as illegitimate.
For many dictionaries, words built from an established root word and a particular prefix fall into a gray zone where the lexicographer must make an arbitrary decision about whether to include the term as a separate dictionary entry, a subentry, or not at all.
For example, Merriam-Webster's Eleventh Collegiate Dictionary (2003) deals with the twelve entries from AHDEL noted above as follows: underachieve (no separate entry, but listed under underachiever), underbid (separate entry), underbuy (no mention at all), undercharge (separate entry), undercount (separate entry), underdo (no mention at all), underdress (separate entry), underemphasize (separate entry), underestimate (separate entry), underexpose (separate entry), underfeed (separate entry), and underfund (separate entry).
Missing from both dictionaries are such words as underact, underbake, undercook, underemploy, and (of course) underfill. That all of these words are in use in published writing is evident from this Ngram: if you click the links beneath the graph you'll see links to instances of the word in Google Books (warning: some of the matches for underact are misreadings of "under act," and some of the other words' matches are to instances where the word in question is used as a noun rather than as a verb, as in the case of undercook). Note that the matches here generally exclude instances where the word is spelled with a hyphen between the prefix and the root word; and note, too, that Microsoft Word does not give the squiggly underline treatment to under-fill, even though most U.S. style guides (including the Chicago Manual of Style) advocate omitting the hyphen from under- words.
My main point is that even American dictionaries published within seven years of each other don't entirely agree with each about which under- words to give full entries to. To judge from the Ngram graph mentioned above, use of underfill has increased significantly in the past thirty years, so I would not be at all surprised to see it gain its own entry in the next edition of AHDEL. But with or without such recognition, it is clearly a word in common parlance that has a consistent, widely understood baseline meaning. And given that reality, dictionaries—and Microsoft Word—will eventually catch up with actual usage.