Hello: It’s pretty common that the mixing valve gets stuck open, so it is always bleeding cold water into the hot water during use. Test for this by putting a finger on the cold line to the mixing valve and have somebody else open a hot tap. If the cold line turns cold quickly, you know it is letting cold in. These valves often get limed up in the open position, so need to be disassembled and cleaned … or I’ve known folks to install a ball valve in the cold feed and leave it closed when scalding hot water is not a concern.
If it were the dip tube, the problem would be year round. 😯