First idea: the service technician lowered the thermostat setting (turn it to 60°C or hotter, otherweise hot water will just not less).

Another idea is that the hot "delivery" pipe, if your tank is wall-mounted "pipes from below" Or the cold "injection" pipe if your is floor type "pipes from above". (note: the longest pipe inside the tank)

Is corroded mid-heigt it causes cold water to be injected mid-height instead of at bottom (or, vice versa, hot water being sucked at the bottom) disrupting the stratification.

Both resulting in percived reduction of boiler capacity (but not causing leakage being those pipe inside the boiler. But this second scenario shouldn't happen with a new boiler.

First idea: the service technician lowered the thermostat setting (just turn it to 60°C or hotter, otherweise hot water will just not less).

Another idea is that the hot "delivery" pipe, if your tank is wall-mounted "pipes from below" Or the cold "injection" pipe if your is floor type "pipes from above". (note: the longest pipe inside the tank)

Is corroded mid-heigt it causes cold water to be injected mid-height instead of at bottom (or, vice versa, hot water being sucked at the bottom) disrupting the stratification.

Both resulting in percived reduction of boiler capacity (but not causing leakage being those pipe inside the boiler. But this second scenario shouldn't happen with a new boiler.