It happened to me also and in time, will happen to you! be sure to have spare PTC, push to connect, fittings at home! As mentioned before by "PhoenixGrey", the 2 Flux PTC fittings are:
Feeder Side = PC4-M5
Printhead Side = PC4-M6
As I went to bed last night, I started printing a 9 hour job. In the morning, I found 150g of coils of filament around the machine as the PTFE tube guide came loose from the top feeder side. There was no previous warnings, the PTC fitting has suddenly stopped grabbing the PTFE tube. I suppose after many many printing hours (about 8 to 10 filament rolls of 1kg ), the grabbing fingers in the PTC wore out. The side pressures of the tube as it moves could be the cause. It took me a while to recover the filament and replace the fitting.
My TIP to avoid this problem: Periodically test the PTC fittings by giving a tug on the PTFE guide tube, if it comes off easily then change it. After my tube came off, I could slide the tube in and out without any effort. I checked the Printhead side and so far it is grabbing well.