I have read about this on Flyertalk a few times - all in T5 days, not before, I will say. Of course that doesn’t mean it’s true!
The story was that bags checked in near enough to departure go straight to gate whereas those checked in earlier go to storage and have to be brought back out once it’s closer to flight time. If very busy, they get stuck in a queue, and don’t make it out.
In my head this has been confirmed by the fact that the one time I’ve had a lost bag on a one-leg journey (no connection) from LHR, was when I checked in a bag early.