This is weird behavior... Tableau should be showing your desired format instead of what is showing.
Would you be able to provide a sample workbook?
EDIT: Steve's approach also forces the formating
Thank you very much for your answers. I will try Steve's approach which will probably work. I have this issue on a German setup PC/tableau. On a US/UK setup laptop I don't face these issues
Oh now I see. This is because in some countries like Germany, France, Denmark... For decimals they use comma "," instead of the dot "." as a standard. So Tableau is behaving normally. Steve's approach would definitely solve your problem.
Hope this helps!