Either unit defective or technician somehow made some mistake while connecting the water supply.

you dont open up the unit when connecting water supply or powering it up. sequence is important for sensitive equipment like that, and usually device like this require electricians.

There is a chance water might go in from the bottom even with the cover on, with high water pressure even easier.

Panasonic also LNE wires bundled together right?

There is a chance water might go in from the bottom even with the cover on, with high water pressure even easier.

Was referring to your incoming, first picture

real plumber will turn off incoming water b4 installing,.. not installing piping with running water,...

Panasonic inside wiring any special?
Why manafacterer never  cover life and neutral wire but not cover Ground (earth) wire?

nothing special. The problem mainly is the outside wire. Water will follow the wire into your water heater if the wire is at high level.

Is the water heater missing ECB? Also why the boiler container is plastic.. Macam not design with safety in mind.
Also the connection of the water pipe source is terbalik.

Left side or right side?

Think 2.5mm² is considered too small for water heaters? They're rated at about 20A. If you use at maximum temperature, I think the wires will overheat.

25A is the maximum current but a 10mA leakage should trigger the trip.

By right, 4mm is the spec
In practice 2.5mm is commonly installed

2.5mm boleh-lah if it’s 20A MCB because it is still within the 21A max current calc of the cable. Most heaters around 14-16A for 3300-3800W. However cannot use highlands model 4800/5200W with 2.5mm ⚠️

I don’t recommend the Schnitzel RCBO because the 25A too big for over current protection if used with 2.5mm cable ⚠️⚠️. Match a 10mA standalone RCD with 16/20A MCB depending on heater size.