Rpi4B needs power off reboot for TV hat

Finally identified something I had been suspecting for some time.
Issuing a 'reboot' command  results in tvheadend unable to find the DVB adapter, but a power off reset finds it.
Anyone got half a clue as to why?
--
"The most difficult subjects can be explained to the most slow witted man if he has not formed any idea of them already; but the simplest thing cannot be made clear to the most intelligent man if he is firmly persuaded that he knows already, without a shadow of doubt, what is laid before him."
    - Leo Tolstoy