I had exactly the same problem.
My sitemap:
Text item=KNX_GV_Living_Screen_9_1_0
Slider item=KNX_GV_Living_Screen_9_1_0 label="Screen living [%d %%]"
The text item shows the percentage correctly. The slider-item does not. I would like to avoid having 2 items for showing and changing the percentage.
The solution I’ve found (thanks to LarsK) was to use the switch functionality in the sitemap.