Can Moonbeam Negate Meld into Stone?

I was playing a druid in a free-for-all with other players. In a moment of near death and desperation, I cast meld into stone to escape from a paladin to try and heal a bit. Because it was a Battle Royale, the DM had earlier decided that I could only stay in the wall for 1d6 rounds. I rolled a 4, so I could be in there up to 4 rounds.

However, the paladin cast moonbeam on the area I entered from. He argued that as a druid I classified as a shapeshifter and would have a harder time with the spell save. I said that since I was in total cover, I would be unaffected. In the end it was two against one in reasoning, and they decided that I was considered a shapeshifter. So I rolled the save, I failed it, and was expelled from the wall. The DM said I didn’t have to take the 6d6 damage from the wall but I took radiant damage from moonbeam.

Maybe I’m just salty, but the way I have always understood these spells says that should have never worked. Can someone help me to make sure I am corrected if I misunderstand?