I’m not sure if my “solution” would work, as I have no experience with any other systems. Our installation is not typical in that the hot water circuit is located in the meter box about 20 m from the house and PV inverter. There is a voltage drop of a few volts when the heater is on (it’s a 20 A heater), and an increase of a few volts when PV export is at maximum (3 kW limited). So a significant voltage swing is possible as heaters and export control systems operate.
The first thing I did was to temporarily turn off the export limit control to see if this affected the heater. I could do this through the service access to my inverter after speaking with the manufacturer and installer. It was very straightforward - basically uncheck a box in the software settings page.
This improved the issue a bit, but after speaking with a Fronius representative they also said it could be due to their stringent grid voltage control systems where output is reduced according to the state of the AC line voltage. They have a number of setpoints where output is throttled down starting at eg 250 VAC and suspended completely at 260 VAC (I think). They said a new version of the software would relax/fix this and other issues, but I don’t think it has been released yet.
In the end I was able to fine tune a couple of the voltage/time limits which eliminated the problem - and also operated the Mk2 diverter in “anti-flicker” mode which takes the average of a lot more cycles before turning the SSR on or off. The export limit has no effect on the diverter operation now, and is kept to 3 kW as required by my connection agreement. The Mk2 diverter works brilliantly.