Most digital air conditioners here in Israel, tends to stop the operation of the fan of the outdoor evaporator unit from time to time when they are used as heat pumps in the winter: https://www.youtube.com/watch?v=Ta58-XRt9oQ
When the outdoor evaporator unit, stop its external fan, the evaporator coil begins to cool down until frost appears on it, and the fan starts again.
Why this is happens?

It is the defrosting cycle, which is started after some time (usually there is just a simple timer which let it defrost either at the moment the heating is switched off by the thermostat, or forced in when the heating is on for too long.

In the heating mode the external unit heat exchanger get frosted after some time, which impact the efficiency. Therefore the unit automatically performs a defrosting cycle, which means the system is switched back to cooling mode and the fan in ghe external unit is stopped. Because the colling mode generates heat that melts the icing and so cleans out the exchanger.

Usually the frosting is hidden inside, only when it is getting heated and so starts evaporating, it may show up on the visible part of the exchanger.

It may also happen your unit has bad reversing valve (it could be slow), so instead of heating the exchanger with the fan off, it is cooling it.
Plus it is normal for the reversing valve to take some time to switch over the direction, it is actuated only by the media pushed through by the compressor (the electromagnet controls just the actuating stream). That means the unit may still continue to cool down the exchanger for some time when already in the defrost mode before enough media is pushed through to actually switch over the reversing valve and really start the main defrosting cycle.

This behaviors isn't related to defrosting, and the unit operated on heating during the time of the capturing.
All A/C here in Israel, stop and starts the external fan intermittently, when they operates on heating, and this is a normal operation. I simply don't know why they doing this.
When the A/C defrost, The external unit make a loud noise of coolant flowing when it switch back to cooling than again when switching back to heating.

I would expect the defrost cycle gets imposed automatically, but then all the related noises will be of course present.

But it could be as well some test cycle, where the controller attempts to determine the amount of ice buildup so whether it should run the deicing or not yet (with no ice, stopping the fan will cause rapid temperature drop in the exchanger; with heavy icing the temperature would be already low with the fan running and large thermal inertia mass in the ice itself, so no significant droop as a response to stopped fan) and distinguishing it from just too cold external temperatures. Never seen such unit yet, but I do not handle them daily, so this feature could be already present in the newer units in use.

All local made A/Cs of Tadiran, Electra, Elco and Amcor from the 90's, had this feature where at heating, the fan of the outdoor unit, stopping, until frost forming on the evaporator coil heat exchanger, than the fan starting again.

Dor in heating mode your basically  running AC cooling outside.  problem is when cooling outside Ice get built
up.  that stop heat transfer from outside to inside. then
fan stop to heat up the outse unit  quickly while in cooling mode to get the ice off the unit.  then it switches
to heating mode with fan running. cycle repeat until room get to temprature