That's strange. I'm seeing here that my camera is running on firmware 100c, but I might be wrong so I'll link an image below.
Yep, that's 100c.
Though, the demonstration video on youtube showed that OP was running it on firmware 100c too and his was working fine. Then again that video is also 12 years old and was running on a then unstable version of CHDK so something must have changed.
I'll try running the unstable version of CHDK tomorrow and I'll post again to see how it goes.
Won't hurt to try, but FWIW, anything that was in the "unstable" 12 years ago is likely in the stable now, and the current unstable isn't much different from 1.6. Also FWIW, if you want to try really old builds to see whether it was working back then, you can get some from the "conserved builds" section of
https://chdk.fandom.com/wiki/DownloadsWe did make a change affecting zoom on many cameras (including IXUS130/SD1400) in 1.5 r5508, discussed on
https://chdk.setepontos.com/index.php?topic=13875.msg143473#msg143473 but I believe this should only affect zoom control from script (set_zoom) not the video unlock. That said, there could be unforeseen interactions between the various ifdefs or something, so if you want to try an old version, 1.4 or earlier would be the obvious first choice.
There could be other camera settings that affect the behavior. Looking back, one of the issues I remembered involved digital zoom: The zoom would stop working if you went from optical into digital. This could be affected by Canon digital zoom or "digital teleconverter" setting (I think these are all a single setting on your cam). I'd suggest testing with that off in the Canon menu, if it isn't already.