Breakpoint Issues

I have tested my demo site in phone portrait mode at 320 px, 360 px and in phone landscape view mode at 800 px, respectively. Loads quite well.

But if I change the phone portrait view mode to 375 px (iPhone 6, iPhone 6S @ 414px, Google pixel @ 411 px) my settings for fixed header seems to change. The same thing happens from within Gridbox when I change the breakpoints to those px settings. See link from emulator - (at 414 px).

See link from emulator at 360 px -

How can I resolve this?

Demo site link is -

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