When NGX pulls your membership from Ramblers, it can do a full sync (everyone) or an incremental sync (only the members who changed). After an incremental sync, NGX wrongly treated everyone who had not changed as "missing from the last bulk load".
That matters, because the "missing from last bulk load" signal is used by:
So both were suddenly full of members who were perfectly fine.
Incremental syncs now keep the complete membership list: they start from the previous list, then add, remove and update only the members in the change set. Only genuinely-removed members show as missing, so the filter and the email targeting are accurate again. Full syncs are unchanged.