I have the same issue.
I have S3 Frontier;
I wll turn off the watch then leave it on the charging cradle for about 6-8 hours, during my sleep hours.
In the morning, I will only see 94% charge.
Software Version Tizen 126.96.36.199 - R760XXU2CRE3
Okay i will try it next time I need to charge -- However, I only had this device for 2 months. It used to charged 100%, the issue just happend when I got the software update. Also, I am using the samung charging brick and cradle that it came with. no obstruction, cable is new, cradle is new.