The 2 Himel 10ma RCCBs seems to be wired incorrectly and serve no purpose. It could be done so on purpose to bypass the RCCB by the contractor because there are shared neutrals between the circuits and they are too lazy to fix it. I doubt they are that retarded to so by mistake.

If the water heater is sharing neutral with other circuit (as in 2 MCB sharing same neutral cable), it will trip due to current imbalance if it is wired to the RCCB or RCBO normally.

To fix this usually have to pull 1 or 2 new cables depending on how it was 'shared' in the first place. If the water heater is under shared circuit, just pull new cables for L + N to water heater especially if it was smaller than 4mm². Just make sure the old cables dismantled properly and not exposed.

If the original circuit has working RCCB + MCB for both waterheaters, it will also mean there is no shared neutral. There is absolutely no reason for it to be bypassed or wired incorrectly unless they mess up the original water heater circuit afterward. It could be as stupid as new ventilation fan neutral from the water heater and live from lighting or vice versa. Rather than guessing, it's better you ask the contractor to rectify it. If they really add more points looped from waterheater circuits just to save cost, fxxk 99 them.