It is stated that If team members have different tiers, the Battle Cup will use the highest tier from the team.
In the past different tiers are able to play together. what is wrong with your coding this time? couldn't search battlecup.
South east asia division 5/6. 19 oct 2019. playing with Tier 3, 4 and 5.

https://clips.twitch.tv/ShakingInventiveSpiderFailFish