External display stopped working in 22.04 with ThinkPad Carbon X1 gen 11 usb-

[thayer-t]

My thinkpad carbon x1 gen 11 had been working for about 3 weeks without problems connecting to my Philips 27" 4K external monitor over usb-c / thunderbolt cable (or an older Samsung 4K). It's running ubuntu 22.04 jammy with X11 from x.org (not wayland, but I've tried both)Now the kernel doesn't see the external display, although it can get power through it and see the display's usb hub and attached devices (according to dmesg).
It's fairly new and I've been updating the system software, so I suspect a firmware or software update has broken this.
Notes:

• I hear a sound when plugging in then an immediately "disconnect" sound. In syslog I see usb hub being detected but nothing explicitly about "display" or "graphics" or "video"
• the cable and display work well with a macbook, and had been working with my laptop previously.
• I've tested with both the laptops usb-c ports
• xrandr doesn't see the external display either
• About graphics: Mesa Intel® Graphics (RPL-U)
• About hardware: Lenovo ThinkPad X1 Carbon Gen 11
• OS: Ubuntu 22.04.4 LTS
• Kernel: 6.5.0-1019-oem
• Kernel details: #20-Ubuntu SMP PREEMPT_DYNAMIC Mon Mar 18 17:38:55 UTC 2024
• plugging in a native hdmi to hdmi cable works on my Philips but not my Samsung 27" where it's flakey and pops in and out.
• current firmware is UEFI BIOS 2024-01-25 N3XET50W (1.25), and the software updater reports I'm up-to-date
• Tested with X11org and Wayland X11

What's the right way to debug this? Best next step? Should I compile my own kernel?
Should I try to reach Canonical (ubuntu), Lenovo (kernel is oem), or someone else?

[thayer-t]

Was working for 6 months after updating OS and switching to Wayland. But a recent update and reboot caused it to fail again. So disappointing...