Tracks/mask points drifting on file load.

Hi all - I hope this is the right forum for this; I don’t think it fits into the others, and I’ve already searched for answers unsuccessfully.
I’ve done a couple of separate camera/object tracking projects, using the last 3 iterations of Blender, and keep running into this issue, and it’s causing me to redo a lot of work. Either I’m doing something wrong, or there’s a flaw in Blender somewhere.
Tracking (object or camera) and masking all both affected. If I have a set of tracks/masks that are perfectly aligned, save the file, close and restart Blender and reload the file, some of the tracks will have drifted off. I just spent an hour or so fine tuning a set of masks to hide tracking markers in a video, and when I rendered it there are patches of the video where the markers flash up, as they’re not covered by the masks. Scrolling through the file frame by frame shows the masks have drifted from the marks they were covering. Not on every frame, but when it happens it’s usually on consecutive frames.
I can replicate it as below:
Open a new file, and select Movie Clip Editor. Open a video in the Browse Movie Clip area.
In the properties panel, click the plus to add an new Object.
Add a track, and Ctrl + T to track it.
Scrubbing through the frames, the track is hopefully on target for however long.
Save the file, quit and restart Blender and open the file.
Scrubbing through the single track created, it now drifts off as the frames increase. Even after 10 frames my example is no longer on target.
This is making it hard to produce low errors on camera/object tracking, and I’m struggling to mask anything like effectively.
On my last attempt at my project I saved as I went, but didn’t reload before rendering the video, and I still had errors creeping in, but I can’t replicate this in my short example above. Possibly it’s not the save/load alone causing this, but I don’t know :confused:

I’m at my wit’s end - am I doing something wrong, is there a workaround to stop this, or possibly is this a real bug?
Please help. Ian.