How to correctly LOCK motion tracking points so they don't move after reloading movie clip? I was trying to stabilize some movie clip and met quite annoying issue in Blender 2.79b, although I recall this was happening in earlier versions too. 

The following image shows an example of certain feature (a detail in the guardrail) which I was trying to track but the feature itself does not matter - this is a generic problem with any tracking point.

So... let's assume I reached the point where I am satisfied with tracking results. The matching was done to previous frame not the key frame, nevertheless the preview image was steady over all frames of interest (not counting minor changes due to perspective and noise). To prevent accidental editing I always click on the lock in the right top corner.



Now I can either restart Blender (after saving project of course) or just click Reload in the Clip section and all my tracking points are affected by little offsets in random frames, even if I have locked them. The image below is an example of such offset - the frame 11 is clearly shifted when compared to frame 10 and 12.



So any ideas what is happening here? Is it possible that tracking points are OK, but movie clip is different each time I reload it? (e.g. shifted/duplicated frames or something...). The movie is in MP4 format directly from my Lumia 640.

Edit: further notes

Just noticed that for certain frames it's even worse - the tracking information is completely lost! I don't need to actually Reload the clip - if I move forward the prefetched frames are removed from RAM (in the Movie Clip Editor there is such violet bar indicating which frames are cached in RAM). If I return to these frames, the tracking points are modified again.
As I remember; from the introduction of the tracker, it was always suggested to export a movie to an image sequence. This is to prevent decoding changes/errors to frames when scrubbing through, skipping and reversing direction, as all these conditions can affect the decoding process.
The symptoms of the problem you describe sound exactly like this problem.
I would suggest you convert your movie into individual frames, and then work on that image sequence.