I have the S6 Edge Plus & mine has been doing that for a couple of months. In fact it deteriorated so much that the battery also got totally drained within 2 hours of charging!!
So I decided it was most definitely time for a NEW phone. After all it was 6 years old & has been used constantly EVERY single day-so it's done well.
So I pre-ordered the S21 Ultra last month & Samsung delivered it 11 days ago. If you like your S6 as much as I did, you'd definitely love the S21 Ultra.