Why would a microwave oven not work properly on a petrol generator?

The frequency is going to be a HUGE issue. Pretty much nothing except (some, not all) switched mode power supplies will work on 60hz. You're probably damaging everything you try on it.

How on earth did you even get a 60hz generator here?

Faulty then. 230/60hz isn't even common - its used in Brazil and a few other places.

The 60Hz is an issue for a start, there is something wrong on the generator setup.

The socket tester result I would be less concerned about. It’s designed for use with a centre tapped transformer and the generator doesn’t have that, so it’s not going to give a correct result.

Those generators aren’t really suitable for electronic loads in any case. You would be better off with an inverter generator. Loncin do a half decent one at reasonable money.

I have one of those under a different name. Its heading for the tip but its heavy and I've not got around to it. Keep thinking I might keep the petrol motor.

Long story as to how I got it for free, but the CE mark is fake the wiring is unsafe and the output is way out often putting out 400V AC.

Circulation pump motor may be running 20% faster than expected on 60hz. This is generally not a good thing for the pump.

The label on the genny isn't worth the paper its printed on. If it has a 220V and a 110V output then if the 110V output is 60Hz which it would be for America then the 220V side would also be 60Hz.

As an emergency purchased DW8500W from DD(new, in the box) advertised as 8.5kW capable, knew its BS from the start, but beggars cant be …. - POS, but got me out for few days
12-15 hours continues run for 2 days = ~€20 petrol per day to run
output ~230-235V , frequency fluctuate 50-55Hz but allowing inaccuracy on my measuring tools
Ran of it freezer, fridge, x2 circulating pumps and one shower pump, few lights (up to 4, 11W each), 4G router, x3 8port switches(one PoE) and x2 AP's and of course charging phones/power banks/18V(3Ah, 5Ah) batteries for makita torch . Even managed to run my PC at same time :D

Problems i had:
once connected to house wiring(sure disconnect ESB side) with more-less all above devices - voltage drop to 205V, fridge didn't like it, didn't perform at all but survived.
at shower pump "kick-in" lights would flicker(obviously), imagine all other devices struggle too

After discovered V drop, reduced the load(devices) and got extension leads instead through-the-house wiring:
Freezer, fridge, x2 circulating pumps, chargers, few lights
Voltage fluctuate 220-229, frequency fluctuate less

Power restored just few hour ago - so far all appliances working OK

Need to lay down "emergency" wiring for future occasions.
Think genny-to-house connection wire was to thin, potentially leading to V drop(continental plug - all i had to hand was 1mm2 lead)

Genny - meh, might shop for a better one.

monitored on first day run as was suspecting heat potential - none by touch . Genny ran in the workshop so - cold environment. checked in-house wiring where could reach - no heat by touch felt.
Wiring distance also could play a role - workshop-to-house run on its own ~ 30-40 m
Anyway, extension reel ~30m directly from genny to fridge + 2m from it to circulating pumps was 1.5mm - no heat and V drop minimal.
Another extension reel from genny ~10m to freezer (again 1.5mm) - same "satisfactory" result.

One thing for sure - generator i got is not capable to provide for entire house, but might cover few "must have"

And need to run something like 4mm link(s) of independent wire from home to designated generator location , to be ready-to-plug.

Usually the 110V side is for British / Irish handheld work site tools, which are 110V 50Hz (normally with a two lives out of phase — centre tapped, so it’s about 55V from either side to earth). Unlikely to be intended for US 120V 60Hz appliances tbh.