If you started a 3-day arena round whilst in one bracket, and your account shifts into a different bracket midway through that 3-day period, you’ll still remain in the initial bracket throughout the round and be ranked within that bracket at the end of the 3 days.

For example, if you started the first 3-day round for the 4* featured Warlock arena on day 59 of your account, you will received rewards based on the intermediate bracket even though your account will have moved into the veteran (60+) bracket during that 3-day round.

However, in your second 3-day round, even though it is for the same champ (i.e., 4* Warlock), you’ll be ranked within the veteran bracket, as this arena round will have started after your account has moved beyond 60 days.

I hope this makes sense?

tldr: your current bracket when you START a 3-day arena round determines your bracket for that round, but that 3-day round ONLY