SUMMARY
It is not possible to connect with the Windows 11 MSTSC client to a krdp server. This is the primary client used on windows, as the other clients on the microsoft store are officially deprecated, and the windows freerdp client is unmaintained. 

STEPS TO REPRODUCE
1. winpr-makecert3 -format crt -path . -n "rdp.local" -y 2
2. krdpserver --certificate rdp.local.crt --certificate-key rdp.local.key -u user

OBSERVED RESULT
[01:09:21:579] [9455:000024fa] [WARN][com.freerdp.api] - [peer_unexpected_client_message]: Unexpected client message in state CONNECTION_STATE_FINALIZATION_FONT_LIST, missing flag FINALIZE_CS_FONT_LIST_PDU [0x00000100]
org.kde.krdp: New client connected:  Windows platform Windows NT
[01:09:21:579] [9455:000024fa] [ERROR][com.freerdp.api] - [rdp_peer_handle_state_active]: PostConnect for peer 0x5bfd60f6ab10 failed
[01:09:21:579] [9455:000024fa] [ERROR][com.freerdp.core.transport] - [transport_check_fds]: transport_check_fds: transport->ReceiveCallback() - STATE_RUN_FAILED [-1]
org.kde.krdp: Unable to check file descriptor
org.kde.krdp: Closing session
org.kde.krdp: Closing Freedesktop Portal Session

EXPECTED RESULT
A successful connection

SOFTWARE/OS VERSIONS
Windows: 11 25H2, 26200.6901
Linux/KDE Plasma: CachyOS, 6.12.55-2, up to date
KDE Plasma Version: 6.5.1
KDE Frameworks Version: 6.19.0
Qt Version: 6.10.0

ADDITIONAL INFORMATION
Unexpected client message in state CONNECTION_STATE_FINALIZATION_FONT_LIST, missing flag FINALIZE_CS_FONT_LIST_PDU [0x00000100] also happens when connecting to the sample freerdp server, I don't think its related.