We tested on several machines, all working correct. The problem is never reported by any other of our clients so I believe there must be something in the settings of your machine.
It is vary strange to me that the decimal separator in your culture is correct and then it appears another one. I don't know why you are facing the issue, and the information is not enough to investigate it.
Try to debug your application further: check if the Culture for the control is the correct one on latest events like Page PreRender. Try to insulate the problem in sample page and see if it will replicate and test in different browsers. Also host the page somewhere and try to open it from different machine seeing what is the result. All the information that might not look relevant at first, can actually help to find out why the problem happens.
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