We had this issue on our html websessions. This impacted both internal and external users. The fix for us was setting a Persistence Timeout of 8-12 hours, and additionally using the Horizon GPO bundle and setting the session timeout for 8-12 hours to match. Hope this helps someone. We also researched using XFF headers to keep the communication to the host Horizon server clean so the traffic didn't go to the wrong server. See the yellow note at the top of this article- Dashboard alerts for unrecognized requests for XML Api protocol connection in Horizon 2209 (8.7) (90...
That issue happens with load balancers on ANY version of Horizon, not just 2209-2212.
For later versions of Horizon be sure to set the Locked.Properties file to your load balancer and server name using portalHost entry.
