I don't know if everyone knows, but you can not receive two Celeste DIYs on the same day, no matter if you go to another island or your own.
Let's say you find Celeste on your island and you get a DIY, then you go to your friends island (no matter if someone is on May or December or whatever) your friend also has Celeste. She will NOT give you another DIY, she will just go thru her normal conversations as if she already gave you one. You, on your file, NEED to TT in order to get another DIY from Celeste.
So this is probably what happened, I had that happen one time actually. I went to a friend's island because he had Celeste and she gave me a normal non zodiac DIY, when I went back to my island, Celeste was paying me a visit and I thought I would get another DIY because hey, I went to another island, but no, she was acting as if I had already talked to her.
It also doesn't matter if you get DIYs out of order, you will still get whatever month you're in Obligatory DIY. A friend of mine TT to February and I got the Pisces lights, and I was still able to get the Taurus bathtub without any problems.
The game just doesn't want you to abuse Celeste by going to different islands, which is why that check is in place. The check goes away the next day, be it by waiting normally or by TT and then you're able to get more DIYs from Celeste.