Ender 5: Bowden tube shifts in/out of extruder during retractions
If it's just the pressure fitting on the brass coupler that shifts in/out, or the PTFE tube shifting slightly in/out but remaining in the coupler, that shouldn't be a problem unless it's so loose it can come out entirely. The blue clips which should have come with the printer are intended to stop this motion or at least provide enough pressure to prevent it from falling all the way out.

Regarding the small 0.25 - 0.5 mm of play, effectively it just means you lose that much length from your retractions, since part of the retraction goes into pulling the tube back into the coupler. When the retraction is reversed, the effective filament position should go back to what it was before the retraction. As long as your retraction amount is sufficient after the loss, it should not cause stringing.

You probably need to look for another cause of your stringing.