Need advice - Chain falls of chainring after unfolding

OK y'all, I need some help.  Ever since I've installed the standard size chainring (and installed a longer chain with 100 links) I've had trouble with the chain falling off the chainring when unfolding "briskly". That is, unfolding fast in one motion.  This doesn't happen every time, but probably about 2/3rds of the time.  The B never did that with the original (reduced) chainring.  Any ideas?

Al from Portapedal said that I might be a tired (worn) chain tensioner and is going to be sending me one to see if it helps.

This is how we do it

So, in anticipation of receiving my bike I've watched these two videos of the folding procedure dozens of times. I'm pretty confident that it is THE best videos to show it.