RE: XPS 9550: Latest Thunderbolt firmware upgrade destroys HDMI port
I am seeing the same here on a Precision 5510 which I think is the same platform as the XPS 15: External DVI screen connected to HDMI via an adapter shows inverted colours after the update (actually it started during the final steps of the update). Disabling Thunderbolt in UEFI does not help.
The new Thunderbolt firmware version is 4.26.03.001 ,A07, released today.
Unfortunately, there is no older version of the firmware available for download for my service tag.