NFC antenna connection to ground midway through loop
From AN1445 - Antenna desugn guide for MFRC52x, PN51x and PN53x

Antenna Symmetry
The symmetry in antenna design is absolutely necessary with respect to tuning and EMC behavior.  Otherwise common mode currents are generated to parasitic capacitances from the antenna to ground.  Theses currents can cause emissions that hurt EMV regulations



It can be seen that the center tap of the antenna is connected to ground.  Basically. we do not recommend grounding the center tap, but leave it floating.  This has the advantage of a virtual ground point which is floating to achieve symmetry of the antenna.

Odds are the antenna is radiating noise to the surrounding environment (as shown in the picture), so they ground the virtual ground point to reduce electromagnetic emissions.  They are releasing a demo board with no concern for EMC emissions.  It is a compromise to placate EMC regulations.
Easy to verify, cut the ground trace and use a radio with antenna to verify noise output.