My 2 cents:
The Constant: No wound
Ji Yeon: Inflamed, fresh wound, with several small stitches visible
The Shape of Things to Come: Older wound, with stitches very visible, yet fewer in number than in Ji Yeon
I don't think this is evidence of time travel. More likely, since they filmed these episodes at different times, the makeup artists didn't replicate his earlier size/number of stitches exactly. I.e., a prop error. Another possibility is that, being in the water for many hours, dead, and decomposing, the wound's appearance was affected. Perhaps the stitches were stretched as the skin deformed around the wound differently than the uninjured skin on his face. Maybe those little fishes that eat dead skin were nibbling like mad at the skin around the wound, prior to him washing ashore, making the stitches more visible again. So many simpler explanations than time travel.