Will the antenna be detuned by the presence of the MHF3 plug in parallel?
UFL connectors when used for RF production tests are placed in series to the antenna PCB traces.
See here:
https://www.megiq.com/images/Content/MegiQ-Antenna-Diversity-UFL.jpg

The mere presence of a small metal object might slightly modify the radiation pattern.
The problem is not the de-tuning of the antenna.
The problem is the impedance Zstub that the stub shows to the RF signal coming from C68.
If Zstub is of the same order of the Z of the antenna then you have a problem because part of the RF energy may go into the stub and possibly reflected back.
To estimate Zstub you need to know:
1. PCB trace parameters: witdh, length, thickness

2. PCB parameters

3. How the stub is terminated. Open circuit?