A Latitude 9420 with Ubuntu 20.04 that is installed cannot switch display mode intermittently when connected to dual QHD (2560x1440)@60Hz monitors on a Dell WD19 series dock.
A QHD (2560x1440)@60hz video signal uses 5.6 Gbps of bandwidth. 2 QHD video signals exceed the 8.6 Gbps of bandwidth of HBR2. Linux only supports HBR2.
This is working as Designed, a work-around is to switch the refresh rate to 30 hz.
More information is available in the WD19 series dock user manual for Linux setting:https://dl.dell.com/topicspdf/dell-wd19-130w-dock_users-guide_en-us.pdf