What is the reason in Wet leg level measurement, both leg is connected?
As you can see I the picture https://ibb.co/mrACik (Lp Heater).
It should not cause any problems.
It is possible, that with the purged reference leg, that the tube connection allows a "slight" purge to protect the lower connection from pluggage or build up...
A DP level transmitter always has a 'wet' connection at the low elevation tank connection.
A wet leg is used for the high elevation connection when the vapors at the top of the tank tend to condense to liquid in the impulse line. When the condensate/liquid collects in a dry leg, it applies a hydrostatic head pressure that is an offset pressure on that leg, creating an error in the measurement.
A wet leg attempts to ensure that there is constant hydrostatic head pressure in the wet leg so a correct measurement can be obtained.
Wet leg measurement is used to build the hydrostatic head pressure to transmitter. DP should be made zero by sensor trim, otherwise it will create additional DP. Generally Wet leg builds the head pressure and error due to condensate can be made zero.
The reference fluid should be the same as the fluid in the tank, otherwise it will eventually get contaminated. If it's something like steam condensate, you should have a condensate pot to ensure the reference height is always constant.
I see no point in having the transmitter mounted below the vessel. It only contributes to errors.