Hot water tank only has warm water after bath
This is normal.
Your tank has 60 litres. You took 120 litres of cold+hot. So, let's assume you took 60 litres of hot and 60 litres of cold water. What went in? Cold water. Heating up this cold water will take time.
The only thing anormal, to me is :

We have a new 60L gas hot water tank.

That's small. Where do you live ? I live in a place with both abundant hydro electricity and abundant water, so a water heater would be more like 60 gallons.
After edits:
Is the dip tube installed correctly ?
https://www.discovercabrillo.com/what-is-a-dip-tube-in-a-hot-water-heater/#:~:text=A%20dip%20tube%20is%20a,the%20bottom%20near%20the%20burners.