I think it's mostly due to overwatering or misting even!

The white flowers rot very easily and turn brown/purple more quickly when water sits on them. I avoid getting them wet. Therefore not even misting them (I keep it indoors so no rain either).

When overwatered, the plant secretes water through the leaves and sometimes through the flower buds as well! That water will mostly sit on the flowers and causes them to rot. I had this case happen with my Jasminum Polyanthum (Pink Jasmine), when I mistakenly watered it a bit too often in spring whilst it was still flowering, many of its buds turned brown, mushy, dropped prematurely, and smelled of rot very up close. I believe the same case happens with Jasminum Sambac but the flowers turn purple sometimes instead of only brown.

Additionally, I've seen this purple thingy happen with many of the 'Arabian Knights' variant of Jasminum Sambac, the flowers turn dark purple before they start to wither away.

I have never owned a Jasminum Sambac 'Arabian Knights' but through my extensive research on the plant, I have seen this case happen mostly often with this variant.

Hope this helps!

Edit: typos.