Party matchmaking stuck / lobby search loops or splits party

Build/Version: Open Beta
Time & Timezone: 16.08, Europe/Berlin
Region: EU West
Platform: PC (Steam)
Cross-Play: On
Party Size: 3

Steps to Reproduce:
1) Create party with 3 players.
2) One stays in game, one goes to menu and stays in lobby screen.
3) Different search timers, party members join different lobbies.

Expected Result:
Party reliably matches together into one lobby.

Actual Result:
Stuck on lobby search, error, or party split.

Frequency:
Twice in Beta

Workarounds Tried:
Change host ineffective, restart game → effective.

System/Network:
CPU: AMD Ryzen 9 9800X3D
GPU: NVIDIA GeForce RTX 5080
Monitor: 3840×2160 @ 240 Hz
OS: Windows 11
Connection: Wired LAN 1Gb/s