content not follows app bar collasping

[piaci]

if using in viewpager, the first opened pager item does not scroll with collapsing layout. app bar collapses but the webcontent scrolls normally.
interestingly, if you scroll away from first item and scroll back, then it acts normal.

[Tijn1314]

demo?

[piaci]

here you go:

demo.mp4

[Tijn1314]

here you go:

demo.mp4

can't watch

[piaci]

let's try a gif then: