I have HUNDREDS of custom brushes. Every time Photoshop updates, be it on my computer or my iPad, they keep getting deleted from Photoshop. (Not to mention Creative Cloud keeps having some issue where it crashes and almost constantly and needs to repair.)
Yes, I have my brushes correctly installed via Creative Cloud's instructions for properly installing brushes. I've not tried dragging and dropping them into any folders anywhere. And I don't think that solution would work at all for this issue happening on my iPad for when Photoshop updates.