I expected the name to be as it appears on the register, not the supplementary info added years afterwards,for it to be included as it was at the time the register was compiled, if other family members are searching for this info and not aware of her remarriage, as many didn't.
I have just rechecked my great aunt, who also has three surnames in the register and she shows up if you search on any one of those three. So regardless of whether you were aware of her remarriage or not, whe would still be findable.
One of my aunts is not findable under her maiden name. They have her indexed under her full Christian name with her Christian name again in brackets and married surname. I cross checked using the folio and piece no. and found her husband and the rest of his family so I know I have the right lady
Seems to be a matter of pot luck.
If I understand you correctly, the index says:
FirstName (FirstName) MarriedSurname
If that is the case, then obviously it won't appear in a search on the maiden name, as it isn't in the database.
Have you purchased the actual image yet?
It could be that is exactly what the original record says, so is not FindMyPast's transcribing or indexing problem.
As has been highlighted a number of times in the various 1939 threads, this was a document containing many millions of names, updated for over 50 years, with interaction by many many people. There is more than enough oportunity for it to be imperfect, before even considering FindMyPast's work on it.
All things considered, it's in pretty good shape.