I also have a similar problem with Digital Crown from Apple Watch Series 2 during calls, but not always. The watchOS version is 4.2.
In my case I finally found what is the main source of problems (my opinion). During a test to listen music from my Apple Watch Series 2 via Bluetooth I noticed the same behavior of volume indicator up/down (low/high glitches) when adjusting volume. The volume indicator (in this case a circle) is displaying erratic information that is interpreted by Digital Crown because of active Bluetooth connection with Headphones. This is a peculiar behavior because it didn’t happened before and it occurs only when Phone Calls are made or when listening Music (active Bluetooth connection).
The Apple Watch was buyed on 30.05.2017 and now I have serious technical difficulties with Digital Crown because of self continuous Bluetooth RF interference.
My wife has a brand new Series 3. Hope she will not get this defect.
I will go after vacation and New Years holidays to an authorized Apple Service. Hope the guarantee from Apple Watches is covering this malfunction.