OHG fust < *funstiz
Our entry for *funstiz mentions OHG fûst as a descendant. So nasal spirant law. Do you have enough knowledge about Old High German to confirm that or can send me to someone who has? German has one or two examples of -unst from PGM sources and even Saxon, on average, didn't denasalise.
It is a bit unusual, but even Dutch has vuist, also without a nasal. However, there may be an explanation. This word is often thought to derive from the Indo-European root for "five", or from a root *pewg-, and in that case the expected Germanic term would actually *funhstiz. And in that case, the nasal would have been lost in all Germanic languages. But the change of nh > ¯h is a very late change in Proto-Germanic, and thus the loss of -h- in this word must have been later still. And yet, it happened in all dialects?