I switched out my original charging puck for the one that came with the S4, and instead of using it on a charging stand I have, left it flat on the bedside table - no repeat of the frequent 100% charged alerts. It could therefore be that the angle of the charging stand meant the S4 didn't sit properly in the original puck, or that the original puck isn't quite the same shape and therefore the S4 didn't fit it correctly. In either case, it could also be exacerbated by something in watchOS 5.0.1 that caused a possibly intermittent connection between the puck and watch to report 100% charge multiple times. Logically, it should not really be an issue with the puck, because Apple make no differential between old and new, and I'd think they would really have to if they were not the same. Also, while this may not mean anything, yesterday, after 1 hour off-charge the watch was down to 95%, and dropped to 93 by 90 minutes, while today, it is still at 100.