Apologies if this has been suggested but I've only scan read the above. Saw my mechanic of choice the other day for a similar perceived problem. He pointed out some bushings on the rear which he said were common for causing some stability problems, perhaps like you mention in your original post.
I believe he pointed at bush number 15 on this image. Said to be these can be damaged when lifting the car as the suspension flexes in the opposite direction to normal operation - causing the bush to become loose in its bore. He tested by prying at this bush with a crowbar checking for horizontal play front-to-back. Suggested mine had some play but didn't warrant replacing yet. Worth a mention to your mechanic maybe? I would point out I may have identified this incorrectly. Though I think this was the area he showed me a few days back. Can be seen if stood under the middle of the car looking towards the forward most components of the rear suspension assembly.
I've decided to keep an eye on my issue and if it persists replace the arms which hold this bush. For now I'm trying outer courses of action to see if I can make improvements. Tyre pressures, adjusting steering weight with VCDS etc. P.s. My set up is totally stock.
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