The following issue is currently occurring in the game.
When multiple effects that lower stats are triggered at the same time through things such as Passive Skills, the effect of Downside Up may only trigger properly for one of those effects. This issue occurs when the stat lowering effects are activated only on the Sync Pairs that have the Downside Up Passive Skill.
Since August 31, 2021 at 11:00 p.m.
We are currently investigating the cause of this issue.
We are currently considering addressing and providing compensation for this issue.
An additional announcement regarding the resolution will be made after the updated version with the fix is released.
We sincerely regret any inconvenience caused.
We hope you continue to enjoy Pokémon Masters EX.
Source: Official Pokémon Masters EX site