(Posting from my wife’s account because I hit the 24 hour limit)
Theory confirmed!
I just changed my monitor’s frame rate on my newer computer down to 60hz instead of the usual 240hz, and the tweens worked great.
If you have a way to adjust your monitor’s frame rate, try it yourself and see what I mean.
So… problem identified, what’s the fix? Clearly I can’t expect everyone to run it on a 60hz monitor, this is definitely a bug with Phaser.