This was a bug related to negative neighbour counts for Weighted neighborhoods. I've committed a fix to the repo. Thanks for reporting!
The main change that might be relevant in 2.8 was a switch to OpenGL for rendering.
overlay-demo - pixels demo minor change
overlay-demo - pixels demo is prettier and faster
overlay replace return changed pixel count from...
10x speed improvement for 5 overlay replace use...
Merge branch 'master' of ssh://git.code.sf.net/...
overlay replace command and demo, breakout.lua ...