The sage advice question is in the compendium. It follows the wording but until now the only reason to consider secrets that way would be a bard who traded secrets out and then changed his or her mind and wanted to trade it back after leveling up again, which is just a hypothetical that I've never seen. Backtracking trading up isn't really a thing.
The gray area is in the magical secrets making the spell a bard spell for that character. I could interpret that as adding it to the bard list for that character, or I could interpret the bard list as strictly "the" bard list as listed and I don't think I would actually be wrong either way as a DM.
One way is counter-intuitive to spell versatility in a big way that impacts a very large portion of the bard spells known by not being able to apply it to one of the major draws to the bard class or losing one of the major draws to the bard class. The other supports magical secrets taken by the bard by treating them exactly like any other bard spell, which is typical of secrets. Which is better to tell the player?
It's another area that WotC should address after feedback regardless. To prevent arguments if nothing else.