The patch has landed in 5.14-rc1. I'm not experiencing the issue with this kernel.
Yes, issue not present with patch applied to 5.13-rc2.
Unfortunately after applying the patch to 5.13-rc1 I still have the issue. New message in dmesg:
[ 12.119562] [drm:drm_dp_get_mst_branch_device [drm_kms_helper]] *ERROR* failed to lookup MSTB with lct 2, rad 00