Same here, i've been experiencing this issue since I installed the game back in August. It is tied to very specific moments in game for me too.
I went the entire ARKS Boost Rally without ever getting to complete an Advance Quest because it would always without fail disconnect me as the the party moved to the boss room. Same story with the Crimson Menace. As soon as I complete the mission, the dragon dies, group photo, about to loot and disconnect. Worst part is not only did I waste my time and boosters but I can't attempt the mission again to get loot because it's Limited to one Completion.
I have asked mods on the Steam Support on the PSO2 Discord and nothing they have suggested works. I tried adding the relevant files to the allow list on my antivirus, I disabled my antivirus entirely, I have run the game in admin, i have downloaded the tweaker then switched back to steam, i have done the file fix on both steam and the tweaker, i have uninstalled and deleted the whole game and re-installed it, I have disabled my IPv6 and cleared the cache, I have changed my DNS to Google DNS. Nothing has worked. I am still disconnecting.