Font size is used to calculate the (min) size of the object and therefor the window so changing it on the fly might cause layout problem.

Easy fixes:

- Use gfx.library functions in MUIM_Draw to render text

- Put the object into a group and only remove/add that object when needed (don't forget to call Init/ExitChange on the group)


Without knowing further details it is impossible wether any of the above applies.