I was having huge flaky thunderbolt problems on my Linux T490 - even updating the laptop firmware didn't fix it despite Lenovo changelogs indicating a bunch of thunderbolt dock bugs fixed.

Then I swapped the Lenovo thunderbolt dock with another one at work and everything just worked fine. I run 3 monitors (2 external 2560x1440) at once, but no 4k.

Maybe it was dock firmware versions - the Lenovo dock updater software was Windows only, so I had never tried it.