I've been noticing it particularly on the return direction of ex-US TPACs (e.g. HKG/SIN/SYD/TPE-SFO), but I think I may have recognized at least part of the issue, and it might be more widespread than those locations.

EF seems to be returning the fare class availability of those flights based on Point of Sale in that location (e.g. Australia, HK, Singapore), despite having US POS selected in the search criteria. (I compared a few flights on EF with what UA was showing on a one-way Asia/Australia-US and then it matched each time)

It does this no matter if I have US POS selected while searching as an ex-US RT, or as an HKG/SIN/SYD/TPE-SFO one-way.