can you please clarify the issue?
you say it blows cold at 16 and hot at 28 - this sounds correct? I may be missing something based on your description. Please confirm.
Please do not forget to click a positive rating as this is the only way I receive credit for my answer,
this is not a common issue and could have many potential causes unfortunately
It could be due to a faulty heater valve, faulty control panel, switch, or sensor.
For this type of issue, to avoid guesswork, the car will need to be scanned and the proper test plans run using a BMW factory diagnostic computer to read all of the inputs and outputs from the system live, and verify where the issue lies.
Please let me know if you have further questions. Please do not forget to click a positive rating as this is the only way I receive credit for my answer, thank you. Best wishes with your repairs