Quote - I thought it was because of the instep actor. If WD assumed the feet to be rigid, it would place them with their lowest points on the ground and further geometry attached at their ends would then be below ground in some positions.

Exactly my thoughts. However I seem to have seen situations where this is not happening, or at least not happening in all frames. It's very confusing. I think the length of the foot geometry, or the position of its end point does play a part. But there also seem to be other factors which I don't understand. I seem to remember that injecting a longer foot geometry did help, in tests I did some months ago, but I don't think it cured the problem completely. Memory is a bit fuzzy on that though. Perhaps I should try that again, but its an inconvenience for the end user.