Difficult Raids 1.19.2-2.0-beta 11
Forge 43.3.0

Once a raid is completed and the bar above displays something like "Default Raid - Victory" (and before the loot chests spawn), applying bad omen on yourself during this period can increase the tier of the raid, and successive repetitions can raise the tier up to grandmaster. It will display "Grandmaster Raid - Victory" to reflect this.

Through this, a player can receive grandmaster loot chests with high-tier enchantments and treasure even though they've defeated only a default raid.

This is a problem especially in modpacks with alternate methods of attaining bad omen, like the Ravager Horn from It Takes a Pillage or the banner-burning mechanic from Savage and Ravage.
It's also a problem even without those mods, as multiple players that had previously acquired bad omen 1 through vanilla methods may be able to enter a village in the victory phase. Each of their bad omen statuses would be consumed, increasing the raid's tier, as well as the (unearned) rewards.

Potential fixes:

• Lock the raid's tier once it has started
  • Prevent bad omen from being consumed and tiers being upgraded after a raid has already begun
  • This will force players to stack bad omen BEFORE entering a village to determine their desired raid tier, preventing the post-victory stacking
• Lock the raid's tier after the second wave has started
  • This is similar to the first option, but it allows players to change the tier at the onset of the raid
  • This gives the convenience of allowing players to begin raids when they're already inside of a village without worrying about being locked at the lowest tier

Sorry for the delayed response! Thank you so much for noticing that. I believe the easiest option is to lock the raid's tier entirely (though I do think locking it after the first wave is a good idea), so I have this planned for the next update.