Cassian and Threepio&Chewie Unique Tenacity Interaction

It's working correctly as coded. It's behaving similarly to omicron Ackbar lead, and part of the reason why getting guard on Drogan was a huge math puzzle with his datacron.

Cassian's Unique states:

At the start of each encounter, Rebel allies gain Protection Up (20%) for 3 turns, Rebel Attacker allies gain Defense Up for 3 turns, Rebel Support allies gain Potency Up for 3 turns, and Rebel Tank allies gain Tenacity Up for 3 turns. If K-2SO is present, he gains all of these buffs.

The code present on swgoh.gg states under Groundwork:

Child effects are applied on: Encounter Start

Encounter start is when Uniques normally fire off in the order of operations.

Threepio & Chewie's Unique states:

If the allied Leader is a non-Galactic Legend Rebel, Threepio & Chewie gain 40% of the Leader's Max Health, Max Protection, Offense, Defense, Potency, and Tenacity at the start of the first encounter, and Rebel allies gain half that amount. Rebel allies have +15% Critical Avoidance.

The code present on swgoh.gg states under I Must Tell the Others:

Child effects are applied on: Round Start

Round Start is the first move of the match (It's also when OG Chewbacca's Guard applies, which is what was causing Drogan Guard math to be so complicated with his datacron).

So, order of operations for those two uniques would be as follows:

Cassian's Groundwork triggers granting allied tanks Tenacity up.

Threepio and Chewie's I must tell the others triggers checking the stats of the leader unit.

Old Ben Tenacity with tenacity up (we'll just use Tenacity up Value for ease of calculation) is 999,900%

Threepio and Chewie is sharing effectively ~200,000% Tenacity with the rest of the squad.

You won't be able to overcome this unless you have unreistable debuffs, or a source of Tenacity down.

It boils down to the Threepio and Chewie's Unique fires off after all the uniques, and even then, it could still be gamified by putting Cassian before Threepio and Chewie in the squad placement.