there is a feature in vanilla... if there is no raiders joined into the wave, that wave would be directly marked as success, and go into the next wave.

so, if a player catches the bad omen and go to a place that cannot summon any raiders (e.g. location y >= 300, only with a dirt pillar, a bed, and a villager), he will directly win the raid

a possible solution i think, to add a tick check before spawning the rewards, e.g. if the tick <= 3 * 60 * 20, do not generate rewards

or override the tick method of Raid class if possible, to fetch the joinRaid's result (a boolean) and check it in the spawnGroup function. if no raiders were spawned, then stop this raid and mark it to failed, inactive