That's exactly what I am looking at at the moment. It's not a bug though, but a simple behaviour decision ( as in trying to read without giving up )...
I have exactly the same "problem", unfortunately looking at the data reported from the gps, this is a gps issue and not MapThis!.
( If you get satellites only in a small cone, the altitude computation becomes very inprecise due to limited bit representation of the numbers. Check how gps devices compute altitude... )
Search for FAKEFEED.
Cheers.