We've refactored how animations work in 0.13 to a shiny new animation graph approach. Super useful and welcome, but there's evidently a regression with how blending from an animation to itself is handled. This is a non-trivial new bug with no workaround: bah!
mweatherley has a great analysis in the comments: it looks like this is non-trivial to properly fix. Hmm. I'll ping the SME-Animation: this isn't something I can bang out in a day next week.