Hey guys I have a little problem.
When I fix an object position, it disappears in Prototype. Take a look:
This behavior is very strange for me.
I already tried to put this object into a frame or make a group, but nothing works 
You can check the file here.
If you have any solution, tell me, please!
Fix position when scrolling is anchored to the X, Y coordinates of the parent frame. Therefore, if your element is fixed behind the fold line, then it will not be visible when scrolling.
You need a feature that Figma doesn’t have yet. You can vote in this thread: Fixed Positioning Starting At Certain Scroll Depth
Hmmm. I see! Make so much sense now haha
Thanks @tank666 
And I voted 
I managed to fix it by changing the constrain to bottom and right
same. thanks for the tip!