Hello Milot Hoxha and Fabiano Albernaz
Even I could not found the appropriate solution for the header flickering yet! What I observe that this happens when the content of the page (height of the page where scroll is applied) is not much enough.

See the difference in these two videos:

1. Very small content (header is flickering):
   https://voicer.com/uploads/yFsEo/1573571488Ionic-header-flickering-on-scroll-down.mp4
2. Large content (header is NOT flickering):
   https://miro.medium.com/max/1280/1*nQDTbexfAt4UBXf3Q3AnwQ.gif
   https://youtu.be/zSuV3XibQ2k

Thanks!