[Snake]

I have a hunch. Dev had optimized the "battle speed" ages ago, as in... Damage happens simultaneously to everyone as fast as possible, instead of what we had which made the game wait a bit while processing each instance of damage individually. This might be related, since you're adding a sudden party member or enemy in the middle of a turn's processing, which causes the game to become confused as of whose turn is it.

I do believe a good band-aid is to make whoever joins the fight in the middle of a round, until the start of the next round:

"On Their Way!" buff, which cannot be stolen nor copied by any means.

- Gain 100% Damage Reduction, +1000 Evade, Critical Evade and Status Resistance while this status is active.
- Set Momentum to 0, and become unable to gain any Momentum (or take a turn) while this status is active.
- Become untargetable by allies and enemies while this status is active.
- Delay their "On Battle Start" effects only happen on the start of the next turn (such as Katana Mastery, Auto Enchant, Auto Summon, etc).

This way it gives the game time to sort itself and acknowledge someone joined the fight, and automatically sort itself as usual.

That aside? It could also be tied to the bug with Nevarious Riff, where an animation is not really clicking with how turn progresses around, which allowed people who are distant to still take their turn after the second Nevarious Riff cast. It was definitely not Nevarious Riff's fault, but Turns being way too "fast" for animations to keep up, I believe. What if same doesn't happen to players?