Problems in changing time signature

How did you add the time signature? When you create a new time signature you need to ensure that its properties (numerator/denominator) are correctly set. MuseScore can read the numbers in Tempo Text, for instance and get the speed correct but you have to explicitly state the properties of a Time Signature.

It looks like you did not actually add a 10/8 time signature. Instead, it appears you added a 6/8 time signature then used Time Signature Properties to make it *appear* like 10/8. That changes the appearance only, though. To actually create a 10/8 or other time signature, use the Time Signature section of the Master Palette as documented in the Handbook under "Time signature".