On December 13, 2021, there were 6 days left until the full moon, and a similar situation happened to me back then as well. If I felt like digging through more screenshots, I could find even more cases of losses against mirror matches. A few times it happened that I lost 7 times against mirror matches, and then another 8 times throughout the entire underworld.
The next day, when I was doing the underworld again, I went through the whole thing without a single loss, without changing any equipment at all. So I tend to lean more toward the “Eurojackpot theory” — or maybe it just depends on how the gods feel when they wake up, since this isn’t Bitefight :D. The chance of success is 50/50 in every fight. It all depends on critical hits, blocking, and accuracy.