I'm statting out a Valor Bard to level 20 (first level Fighter, then 19 levels of Bard.) This means I should get 6 spells from Magical Secrets, of a total 22 spells. Adding the 4 cantrips, this means I should be able to choose 20 "spells known", and have the 6 chosen Magical Secret spells appear automatically. However, I find I can't add my last Bard spell, which is strange since I've counted, and I do NOT have a full 20 "spells known" (it also says I only have 19 known.)

So I go to the Character Sheet page...

My character sheet is showing I've had 7 spells added from Magical Secrets! The extra spell is Wind Walk, which I never picked, for either Magical Secrets OR a normal Bard spell. But clicking on its line brings up the details for the Wish spell (which I DID pick) in the side bar.

I would post a link to your character sheet for the mods. That way they can look into it. 

Thanks. Link is: https://www.dndbeyond.com/profile/RodTheBard/characters/5302906

Just as an update....

While the original character that sparked this has been deleted, I've re-tested the issue on another character. Perhaps it was just a strange fluke, but regardless, the problem seems to have either been fixed or just gone away.