If anyone still has this issue, here’s how I experienced and ultimately resolved it.
I copied a collection of items from one Miro board, navigated to another Miro board, and tried to paste the copied content into it. It would show up temporarily, and then disappear.
I thought maybe it was because I no longer had the other Miro board open, so I opened a new tab, opened the board I was copying from, copied the content again, then went into the tab that I wanted to copy the data to, and pasted it. It worked after doing this. So keep the board you’re copying from open and paste into a new tab with the board you copying to.