Can't add Fizban's Platinum Shield to a level 14 bard through the level 14 magical secrets, despite owning the spell and being the DM of the campaign. Is there any workaround for this, any way to get the spell assigned?

Do you own the character? If not you would need to activate content sharing for the campaign so that character can access the spell.

That's what's strange, I've never had to activate content sharing before to give my players spells, I just added one to a warlock/rogue without doing that (to my knowledge). Is this a magical secrets exclusive thing?

The spells you added before must have been from the list of free content spells.

 That's where the confusion comes in, the spells I had to add to the player's sheets were spells they explicitly could not add for themselves, otherwise they wouldn't have asked me to do it for them. 

There used to be a glitch a while ago that allowed DMs to add things to players’ character sheets that the player should’ve had access to. Maybe that’s what you are referring to.

I am the owner of the Bard in question- I have Steel Wind Strike from my 10th level magical secrets, added back in december i believe. I dont think there was any glitch, Shoddy here just added the spells that he bought to my sheet. He can still add spells to my spell list that he owns, its literally just magical secrets, how does that make any sense? It was consistent before with spells being present and able to be added. 

If you didn’t have access to the spells and he did and could add them for you that was a glitch. That is not intended behavior.