At some point in a Minecraft day, villagers who have not been traded with will completely switch their trade options as if their work station had been replaced.
If you were to find a nice trade at a village, then go to collect the resources, you might come back to a completely different trade than what you had before.
Just in case, same or similar happen in 1.15.2 (so this is not a newly appeared issue), though I didn't check if it happened specifically daily.
EDIT: "If you were to find a nice trade at a village, then go to collect the resources, you might come back to a completely different trade than what you had before."
On the other hand, if you were to find a crappy trade... you might come back for a better one 😉