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I don't believe this issue will be addressable individually so am putting it here for your technical teams to review.
A consistent issue with desynchronisation is happening since implementation of 1.7.1 for multiplayer campaigns in 3 Kingdoms.
A new campaign was started last week, it is not from a campaign that pre-dates the patch.
No mods are being used by host or client.
Error is intermittent in nature, occurring during end turn events (either misaligned end turn sequences, ie client being halfway through the faction list while the host is awaiting players at the end of it) or upon selecting options ranging from diplomacy to the new intrigue mechanic.
Sending host save back and forth from host to client and overwriting the potentially desynchronised version does alleviate the issue but error will reappear after a fairly short period of time, typically within 2-3 turns.
Host computer has attempted to clear certain interfering services that were potentially impacting the network (Killer aps, some windows overlays using network bandwidth) and has reset routers and other configurations have been refreshed to ensure optimal network setup.
A separate campaign has been started in Warhammer 2 which has not had any desync issues as of turn 50, potentially ruling out a broader fault with whatever methods are used in multiplayer campaign online implementation.
I would speculate that the issue is in regards to host/client trying to layer in the new intrigue mechanics through whatever means that feature was put in place but will fully admit my technical ignorance on networking features and how they work for the multiplayer campaign host/client transfer of data.
Side note I did attempt to link the save file to this post but it isn't accepting the file format. If anyone from your team is looking into this and would like to obtain the save to assist with QA / Troubleshooting send me a PM or request it here with an SOP on how to attach the SAVE_Multiplayer file and I will happily do so.