Join measures should inherit breaks and takes properties from last measure
If two measures are joined when the last measure has a system break, this is lost when the measures are joined. One would expect such properties to be kept. The same goes for things like double barlines, etc.
Also, which is more serious: if two measures that are followed by a change in time signature in the following measure, the joined measure gets the time signature of the following measure, it does not keep the current time signature, as one would expect.