The Mecanim Debugging thread might be able to help you track down the issue, but it sounds like you’ve already investigated pretty thoroughly and I wouldn’t really expect anyone else to be able to help much just based on your description.
It could well be a Unity bug so you should create a minimal reproduction project to report it.
In the meantime, you might be interested in Animancer (link in my signature) which is much better at simply doing what it’s told. Also, Animancer Pro gives you access to its source code so you can properly debug it if you have problems.