But even if that's the issue, given that you're mixing a Lenovo dock and a Dell system, I doubt either company would spend much time debugging it. For example, I have Lenovo's USB-C Dock Gen 2 dock and it required a firmware update before it would work properly with my iPad Pro when I tested it out of curiosity.

Otherwise, it may be some firmware interoperability issue that affects that specific combination of equipment. The ideal would be to make an image backup of your system and perform a clean install of just Windows and the required drivers to see what happens, but that's a fair amount of work. I don't know about any issues offhand that affect Lenovo's 40AC (Gen 1) Thunderbolt 3 dock.

Dell's own Thunderbolt dock had a conflict with older versions of Citrix Receiver such that USB ports on the dock didn't work properly when that application was installed (KB article here). Did you try updating the dock firmware, available on the Lenovo site here? If that doesn't work and you're already using the latest driver set also available on that site, then there may be some conflict with some other software on your system.

Lenovo docking station firmware update