Strange u-boot behavior

For some time now I've been booting an Rpi3B+ using a microSD
card configured with 
U-Boot 2019.10 (Mar 17 2020 - 22:01:14 -0700)
by stopping it at the u-boot prompt and issuing
usb reset
followed by 
run bootcmd_usb0
after which the usb hard disk boots and all is well.

Lately, usb reset has taken to reporting
resetting USB...
Bus usb@7e980000: scanning bus usb@7e980000 for devices... Device NOT ready
   Request Sense returned 02 04 01
5 USB Device(s) found
       scanning usb for storage devices... 0 Storage Device(s) found

However, usb tree reports
USB device tree:
  1  Hub (480 Mb/s, 0mA)
  |   U-Boot Root Hub 
  |
  +-2  Hub (480 Mb/s, 2mA)
    |
    +-3  Vendor specific (12 Mb/s, 90mA)
    |    FTDI FT232R USB UART AM00FGTR
    |  
    +-4  Vendor specific (480 Mb/s, 2mA)
    |  
    +-5  Mass Storage (480 Mb/s, 0mA)
         ASMT ASM105x 12345678D558

The problem isn't wholly new; the usb reset command sometimes had
to be repeated once or twice before the disk was found. Now it seems 
a persistent failure. 

Once FreeBSD has booted, the disk is seen and accessed as usual. 

Thanks for reading, any hints appreciated!

bob prohaska