The Universal LNB is an analogue device. It starts sending signals down the cable as soon as it is powered.
The Unicable LNB is a digital device. From a cold start it needs to boot before it can interpret any instructions sent to it by the STB. It has to do this every time power is removed. i.e. every time the tuner stops sending power up the co-ax.
The Unicable LNB is a communal device. So normally it would have an injection power supply to keep it permanently awake. Even if it didn't have a dedicated power source it would always have some user or other connected, so it would always be powered. That means that any new user coming out of standby would be able to use it instantly.
In your case you are the only user and you do not have a dedicated power source, so coming out of standby, or coming from a terrestrial channel the LNB has to cold boot. The cold boot is the reason for the brief tune failed message.
If you had other active devices using the same LNB when you brought your STB out of standby the tune failed message would not display, and the service would be viewable immediately.
The reason this doesn't happen with the Solo4K is because "ForceLNBPower" means the LNB is permanently powered from the STB even when the tuner is not in use.