FWIW - I've used Samsung phones as my sole alarm clock for years..hasn't let me down. Which is not to diminish your problem... occasionally quirky things happen on only a few phones due to various factors unique to the particular phone, os version, other installed programs, settings etc.
Out of curiosity, in the full list of quick toggles, is "blocking mode" enabled? That feature provides a programmable time schedule for turning ringtones and notifications off which sounds vaguely similar to what you're describing. Other than that I don't know of any built in features that would reduce the volume. There are plenty of third party apps to set silent based on time, location, appointments,. Perhaps a clue might be if there is a pattern to whan the phone changes to silent... the same time every day or the same location or in some way related to phone calls, other apps etc.
I have heard a fair number of complaints on the forums of people experiencing various things after the 4.3 update (not me). It is well known among people who flash custom ROMs that best chance for success is to do a thorough reset, so maybe not completely surprising that the official OTA updates (which does not force a reset) sometimes come with problems. You might look closer at what apps you had installed prior to the reset...maybe one of those is not completely compatible with the new os. Maybe trying clearing cache. Last resort (since it can be difficult to set your phone back up) but highest chance of success in general for unknown problems - Factory data reset.