Why Beatgrids Drift, and Why Marking Beats Cannot Fix It
If your grid lines up perfectly on beat 1 and is visibly wrong by the second chorus, there is nothing wrong with your software. The song is drifting, and a fixed grid simply cannot follow it.
Marking is not moving
DJ software and DAWs only mark where they think the beats fall. The audio underneath never changes, so the error compounds the longer the track runs. Dynamic and variable grids just add more marks to a moving target, and every program you open the file in has to guess all over again.
That is why a grid can be perfect at the intro and half a beat out by the outro. Nothing drifted except the music, and the marks were only ever a description of it.
Fixing the cause instead
BPM Lock rewrites the timing of the audio itself, so the beats genuinely fall on a steady grid. The corrected file is permanently on the grid in Rekordbox, Serato, Ableton, Logic, anywhere, with nothing to re-analyse and no markers to babysit.
Every beat is detected individually and moved, so bar 40 is as accurate as bar 1. It is not an average tempo laid over a wandering performance.
How this differs from warping in a DAW
DAW auto-warp guesses a grid, struggles with human-played music, and the result only exists inside that one project. BPM Lock renders a new audio file that is permanently locked to a constant BPM, so it drops into any DAW or DJ software at bar 1 with no warp markers and nothing to re-analyse.