I have received a reply from tech support on this:
"Thanks for contacting Wolfram Technical Support and for taking the time to send in the report. This is indeed an issue of our Interpolation function and is caused by the TetGen, which fails to generate a mesh with sufficient quality. I've forwarded your message to our development team and I will keep you fully posted on any updates and/or workarounds of this problem."
So this is indeed an unexpected behavior.
Best,
David
