I got master tier so my little brother could make a character and play without having to buy the same books I already bought, while still having access to almost all races feats and backgrounds (I wasn't sure how much content the DM owned but knew I owned more). At first we tried making the character in my campaign before moving said character over to the campaign we would be playing on, but he was still only seeing the basic free stuff. When we tried to make the character on the main campaign thinking master tier needed time to activate, it was the same issue he couldn't access anything from the DM's master tier. To be able to make the character with actually options, he had to start it in my campaign, finish it on computer as editing as the "DM", then I had to kick him out of my campaign because he lost control of said character for some reason  and couldn't leave the campaign or do anything with it himself.

At least one of the two master tier's that were owned should have been able to share the DM's content when my brother clicked to make a character for their campaign...

Everyone you mentioned in your post should make sure that their Account Entitlements are in sync: (https://www.dndbeyond.com/account/marketplace). If, for example, your Entitlements got out of sync the system wouldn’t know what to share. More likely is that when your brother joined the campaign and suddenly got access to your content, his Entitlements didn’t sync to the new content so it just didn’t register for him.