Why does a soldered IPEX MHF4 (WIFI card) connection result in signal attenuation?
It is quite normal since the 2.4GHz is… very sensitive to layout and changes of impedance.
First of all the uFL connector is horribly small but it always has two conductor, in  the form of a microcoax cable. It is extremely small but there is a distinction between the center and the shielding.
The other issue is that the connector is precisely manufactured to keep a 50 ohm impedance at some frequency range. We are talking about micron precision manufacturing. Even if you successfully solder both the signal and the return conductors (good luck with that given the size of the connector pads) it will be probably not a 50 ohm impedance connection, and that will degrade the performance.
The correct repair would be to replace the broken uFL with another one and replug the antenna cable. If the pads are damaged the repair is of course a lot more complex.
By the way: the coax length is not critical, it just need to match the 50 ohm system.