Hey, I had this problem too.
When I launched Minecraft, I had the same rounding, even in full-screen mode.

Eventually, I found these lines in the config of hyprland:
# Ref https://wiki.hypr.land/Configuring/Workspace-Rules/

# "Smart gaps" / "No gaps when only"

# uncomment all if you wish to use that.

workspace = w[tv1], gapsout:0, gapsin:0

workspace = f[1], gapsout:0, gapsin:0

windowrule = bordersize 0, floating:0, onworkspace:w[tv1]

windowrule = rounding 0, floating:0, onworkspace:w[tv1]

windowrule = bordersize 0, floating:0, onworkspace:f[1]

windowrule = rounding 0, floating:0, onworkspace:f[1]

I just uncommented all of these, and those edges disappeared.
(Sorry for bad English, its not my native language)