How does slipping Bowden tube affect retraction or does it at all?
The slipping does result in lost retraction distance. It does not result in underextrusion, lost material (except possibly via having insufficient retraction after the reduction), or anything like that.

If your retraction is set to 6 mm, but the bowden pulls 2 mm into the coupler when you retract, those first 2 mm of filament motion do not pull the filament out of the hotend at all. The position of the filament relative to the tube (and thus relative to the nozzle) remains constant. After the tube can be pulled back no further into the coupler, the remaining retraction pulls the filament back through the bowden tube, for 4 mm of retraction at the hotend/nozzle.

When unretracting, the reverse happens. The first 2 mm of extruder motion push the bowden tube out of the coupler, and don't move the filament relative to the tube (or the hotend). After that the next 4 mm push the filament through the tube and back to the nozzle orifice.

The result is the filament ending up back exactly where it started, but having backed out only 4 mm from the nozzle, not the requested 6 mm.

These numbers are just examples but probably about right. If you put the plastic clip on the pneumatic coupler, the slipping should stop, but you can also just increase retraction if the amount after the loss to slipping is not sufficient.