Open my codepen and click anywhere in the frame. It sets the progress to 0.99999 which trigger onUpdate fine, then it pauses at 0 and in this case the onUpdate not called, but I think it should as the background color changed back to red. If pause changes CSS properties then it should give an onUpdate callback too, don't you think? So with this, I'm unable to get my custom property when the playhead sets with pause.
This works fine:
tl.progress(0.9999).progress(0).pause();