It was the same with my previous Desire HD. On the One it does not work, just tested it. I don’t use the alarm function after the problem with Desire HD.
I have observed and encountered glitches in HTC updates. There was an issue with Bluetooth with 4.3 but was resolved in 4.4 on the One.
Your option is to bring this issue on the alarm to HTC’s attention citing that something that worked before but does not work after an update. Hopefully they listen.