The flexM1 has had its fair share of issues. The problem is the interface between the wire wound core antenna and the chip. This is difficult to get good quality consistency, and past flexM1s have had plenty of failures and warranty replacements.
The odd capacitance of the chip makes is difficult to create a PCB antenna solution, and the COB (chip on board) approach used by the Chinese manufacturers of the magic chips makes adhering such a module to a PCB antenna difficult as well.
For now, I have some relative confidence that the current manufacturing process of the flexM1 is robust… but I’m looking again at attempting to get a PCB antenna made for it.