U-boot for FreeBSD on Raspberry Pi

For the last few years I've been using FreeBSD on Raspberry Pi
2, 3 and 4 computers. As it happens, U-boot is employed to
bootstrap the systems and until recently it worked acceptably.

Lately, it's been necessary to tune u-boot via an environment
variable called usb_pgood_delay to ensure successful USB hard
disk detection. Older versions of u-boot could save environment
variables to the FAT partition, so it was a one time change.

Lately, the version of U-boot furnished as part of the installer image
and also (naturally) the ports system omits the environment save command.
On issuing the "saveenv" command the system reports
..."saving to FAT failed".

Does anybody on this group use u-boot and have insights into what might
be done to fix the problem? There's a huge amount of literature online
regarding u-boot but very little that pertains to making "saving to FAT"
work correctly.

Thanks for reading,

bob prohaska