We bought geyser of Indesil (supposedly an Italian brand), which had American thermostat according to them. Later checking after a year revealed it to be of Tesla due to a malfunction.
We got it repaired two times. Now we just dont turn off the geyser all year, it is on pilot and the burner also burns slighlty on pilot.
If you can afford instant geysers for separate bathrooms you should take that route. They work very well and many people can use the shower at same time.
TL;DR: Check the thermostat thoroughly.