The network is not stable enough in customer's environment. Session Reliability and Auto Client Reconnect(ACR) policies are enabled by default. In some scenario, end user failed to reconnect while network recovered.
We recommended customer to disable ACR as a workaround. Customer responded the issue did not occur after disable ACR, but still wanted the benefit of ACR, so requested Citrix to fix it.
Citrix log shows ACR reconnected successfully, but Terminal Service throw an exception code (ERRINFO_IDLE_TIMEOUT), that caused ACR failed.
We did not know the root cause of why Terminal Service threw an exception. from Citrix side, we have implemented a new feature for ACR of slow reconnection. it will fix this issue. the new feature will be released in future release.
CVAD - ICA session failed to reconnect while ACR recovered in Windows Server VDA
https://learn.microsoft.com/en-us/openspecs/windows_protocols/ms-rdpbcgr/a21a1bd9-2303-49c1-90ec-3932435c248c