As far as I can see the problem always lurked in the redefinitions memoir does to the internal output routine commands of LaTeX. You can also produce it, for example, with your example and the old footmisc from 2011, if you add the option "bottom" to footmisc.
The problem only became more appearent new due to the new footmisc being based on updates to the internal LaTeX output routine commands. They are currently only in footmisc, but will eventually make it to the kernel itself.
For the moment you can combine memoir and footmisc, by using the old footmisc release, i.e., by using
That is obviously not a permanent solution because it takes footmisc back to its 2011 version (including the bugs it had then), but it should get you the same behavior as before for now.
Midterm when the new output routine ends up in the LaTeX format, memoir needs to change its behavior and instead if flately overwriting parts, use the (then) available interfaces to implements its version.
By the way: it is not reledmac + footmisc that are incompatible. Your example works perfectly well if you use a different documentclass, say article or book with your example. So please update the title of this question.