Perhaps, the sleep settings, hardware failure, installed third-party software, etc. on your Mac 2021 are the reasons why the USB hub keeps ejecting from your Mac.
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According to your description, the Mac 2018 with the same macOS Monterey and Windows PC both work well with the USB C docking station, the problem should be related to your Mac 2021 when the USB hub keeps ejecting from the Mac. You can try to troubleshoot this issue on your Mac 2021 with the methods below:
Check the USB port on Mac 2021. Given that you use the same USB cable when connecting the hub to another Mac 2015 and Windows PC, if there exists a connection issue, it should be the USB port on Mac 2021. Change to another USB port and connect to another external storage media to ensure you are not connecting to a defective USB port.
Restart your Mac. This process will terminate all running applications and processes and give the Mac a fresh start, it may eliminate some system-related errors that cause the USB hub keeps disconnecting.
Uninstall recently installed apps. It may be the newly add apps on your Mac prevent the USB hub from connecting. If this issue happens right after you install certain apps, uninstalling them could get the USB hub to work again.
Verify if any software is missing. Ensure that you have the most recent software for your device installed. For any updates, check the manufacturer's website.
Ensure that the hub and the device have the same speed. Connect USB 2.0 Hi-Speed devices to a USB 2.0 Hi-Speed hub, and so on. Connect USB 3.0 SuperSpeed devices to a USB 3.0 SuperSpeed HUB.