RPi3B+ with externally powered USB interface sometimes does not reboot cleanly...

[nettings]

... or only after a long time. Check if back-powering is an issue.

[nettings]

Might not be related, but (from /lib/firmware/raspberrypi/bootloader/release-notes.md):

2020-08-10 Promote 2020-07-31 release to STABLE

• The USB port power management change from the last BETA improves
  compatiblity for devices which during reset with no regressions reported.
  Make this the latest stable release.

2020-07-31 Standardize USB port power control accross board revisions - BETA

• Turn off USB port power for 1-second regardless of boot-mode. This appears
  to resolve an issue on R1.3 and older board revisions where some USB
  devices would fail upon reboot. On R1.4 USB port power is turned off
  automatically by the PMIC so this is just held in reset for longer. For
  earlier board revisions the USB port power is explicitly turned off via
  XHCI.
  This can be overriden via USB_MSD_PWR_OFF_TIME in the EEPROM config.
• Update to the latest Broadcom memsys FW - no significant functional change.