I showed him how the hertz increased under load and he would give it some thought.
Not sure if you are referring to Growatt or Trace inverters for this comment.
Hybrid inverters may do this when input AC breaker is just opened.
Open AC input is toughest for hybrid inverters to detect since the pass-through relay is engaged with the running inverter is connected to AC input so when grid AC input breaker is opened, the AC input on inverter still has AC voltage fed back through pass-through relay.
It must determine there is no AC input by lack of AC input current draw or out of range frequency wander. Since zero AC input current is a valid input state under certain conditions (like no load on inverter) it may take some time before inverter releases pass-through relay when you switch off the AC input breaker.
During this period, the inverter may wander off frequency since it does not have grid to keep it in phase sync. Either one of two things happen to cause inverter to recognize there is no AC input connected and release pass-through relay, also causing inverter to move to its internal center AC frequency. One is inverter drifts beyond its AC input frequency limit, other is a current detected on AC input that does not fit the inverter/AC output load condition.
Since grid has tight tolerance on its frequency it is better to keep its AC input allowed frequency range tight to get a quicker pass-through relay release.
A true grid interactive inverter that allows grid back feed will purposely attempt to wiggle its phase periodically looking for AC current wiggle that will result if a valid AC input is present which will resist any inverter phase wiggle attempt. If there is no resistance to the phase wiggle, then the inverter declares the AC input invalid (disconnected or high impedance) and releases the pass-through relay.
On my Trace inverters if I double switch the input breaker off, then back on, then back off again within a second or two, it will immediately release pass-through relay. Inverter then slews back to its self-controlled center frequency.
Normally when grid collapses it drags down the AC input causing an overload on inverter which then immediately releases pass-through relay.