Strange USB loop

After updating to 
FreeBSD 13.0-CURRENT (GENERIC) #5 r364475: Mon Aug 24 06:47:29 PDT 2020
on a Pi3 it was necessary to disconnect the mouse, keyboard and usb-serial
adapter to allow the machine to mount root from USB via a hub.

Once the machine came back up with root mounted from USB, I tried plugging
the serial adapter, mouse and keyboard back in via the hub. 

The FTDI serial adapater was recognized without trouble, but when the
elderly Dell mouse was connected, a stream of

uhub_reattach_port: giving up port reset - device vanished
uhub_reattach_port: giving up port reset - device vanished
uhub_reattach_port: giving up port reset - device vanished
uhub_reattach_port: giving up port reset - device vanished
uhub_reattach_port: giving up port reset - device vanished

began to scroll on both the monitor and console. Unplugging 
the mouse made no difference. Plugging the mouse directly
into the Pi's USB port allowed recognition and function,
but the stream of errors persisted. Network access seems
normal.  

It looks almost as if there's some sort of infinite loop
running in the USB software. The need to disconnect mouse
and keyboard to permit mountroot to work isn't new, but
the "giving up port reset" _is_ new at least to me.

Are there any experiments which might narrow down what's wrong?

Thanks for reading,

bob prohaska