I think the hot/cold pipings are for storage heater. Apasal install instant heater pulak?

Also, the shut off valve stops the water inlet or water outlet? If water outlet, it's dangerous as instant heaters iinm don't have pressure relief valve.

Instant water heaters only have one cold water inlet pipe. You regulate the water temperature by toggling the set temperature switch/knob.

The isolation valve is located at the inlet of the heater. The outlet of heater is not supposed to have any valve to avoid over pressuring the heater and cause internal leakage.

Also, you're not supposed to use steel hose or pipe to connect to the heater to reduce risk of electrocution.

The heating coils are not supposed to be subjected to any pressure by having the water stop valve at the inlet and no valve at the outlet.
I highly suspect TS' connected his instant heater the way storage heaters are connected. The difference between them is storage heaters have pressure relief valve and instant heaters don't (or I've yet to see any). So any overpressure in instant heaters will cause internal leakage as in TS' case.

The part you circled should be the flow switch which switches on the power to the heater and cuts off the power when there's no water flow.

The high supply water pressure should not be an issue because you can regulate the flow (hence the pressure) by using the inlet valve.

As I said, if you installed a stop valve at the outlet (hot water side), you risk over pressuring the heater coil.

Adui, I have stated so many times instant water heaters aren't supposed to have a water stop valve at the outlet.

Your configuration comes with a valve at the outlet line (hot water) to stop water flow, correct?

If so,
1) get it removed and reroute the piping to add stop valve at the inlet, or

2) change to a storage heater

And

3) don't use steel piping or steel hose to connect to instant heater heater.

You are lucky your heater circuit presumably has a 10mA RCCB since it's Singapore, else you upcar liao.

The filter is located at the inlet. If that filter is clogged, leakage would be at upstream of the filter which is outside of the heater housing.

For every complaint about this brand, ayam sure there are 10 praises for this.
How long a heater lasts depends on installation, usage and water quality.
Users who full blast water flow will naturally need to increase heating rate, shortening its lifespan.

A bathroom with more users will naturally wear out the heater faster.

A house with "harder" water will increase deposits lay down in heating coil, causing hot spots and poor heat conduction and thus users need to increase heating power, shortening its lifespan.

It's called storage heater. I already mentioned this but this TS macam keep on trolling.