There are two power manager in your system, Linux and Thinkpad power manager. Do you check both? Normally, you should check something could be relative to sleep time. Some scheme could cause a indirect sleep.

Where should I look for Thinkpad power management settings? BIOS? I checked the power settings in BIOS and didn't find anything related to USB or sleep. The options were:

Timer wake with battery operation: Disabled

Intel SpeedStep technology:
-AC: Automatic
-Battery: Automatic

Adaptive thermal management:
-AC: Maximize performance
-Battery: Balanced

CD-ROM speed: Normal

CPU power management: Automatic

PCI bus power management: Disabled

PCI Express power management: Disabled

Power on when AC attached: Disabled


The card is in a Mini PCIe slot even though it appears as an USB device so I thought it would be useful to switch off all PCIe power management functions.

The Lenovo power manager for Windows has lots of options but could there be something that could not be adjusted in BIOS (or Linux)?

The card is in a Mini PCIe slot even though it appears as an USB device so I thought it would be useful to switch off all PCIe power management functions.

That's good point. You can adjust adaptive thermal management and CPU power management. If temperature inside Thinkpad is high, some device could be shut down automatically to cool down temperature. CPU power management is relative to it, too.
And SpeedStep option is relative to temperature, high speed means high temperature.