I had / have issue with Axure RP 9 the last days and found a workaround to mi solution.
This is a copy of
Axure RP 10 Beta
I found out, that only headers (H1, H2, etc) can have a dynamic width.
Therefore, the header must be styled in the preferred way and then be saved as a new style element.
And you never ever be allowed to click on the size constraints (fit to text width/height).
This procedure must be done for each state of the button (hover/active/disabled).
Also the state styles must have no info about changing sizes, accept for border strength.
I hope this can help some one.