Continuity mistake: There were only two serums left after Morbius took his. Milo takes one and kills the nurse. The next morning when Michael thinks he had killed the nurse he goes to grab a bunch of blood and make a run for it. There are still two serums, even though Milo has supposedly already taken one of the remaining 2.
Continuity mistake: At the beginning of the flashback in Greece, the pieces on and around the chessboard change. When we first see the board, most of the white pieces are on the back row, and then we see 2 pawns removed. In the next shot, when the king is knocked over, some of the white pieces on the back row have moved and there're several pieces that have been removed.
Suggested correction: If you look it's been clear that Strange's spell wasn't perfect. Eddie being brought into the MCU when he had no idea who Spider-man was...nor did he die. Electro's new-look and having no idea who Spider-man was. If anything Clea, in MOM basically said he messed up. So that last spell didn't fix things. It's clear this is what they are building towards. Nothing is fixed. Either way it isn't a plot hole for this movie. Post credit scenes are usually teases for other movies to explain.