To reproduce:
1) Open attached project
2) Expand the hierarchy to the maximum possible
3) In one of the Item Containers, start disabling Items one by one and notice what happens with each disable. Then reenable them back one by one and again notice the changes

Expected: The parent fitter (Main Container) immediately resizes to the new amount of elements in it with no glitches
Actual: When disabling elements, Main Container Fitter always thinks there is one more element than disabled. This means that on first disable fitter doesn't do anything, on second disable it resizes to N-1 elements, not N-2 elements as it should. When reenabling, same problem happens, making the preview look squashed without any spacing. Both of these issues are only cosmetic - on launching the game, the fitter fixes itself

4) Play the scene
5) Repeat step 3

Expected: The parent fitter (Main Container) immediately resizes to the new amount of elements in it with no glitches
Actual: When disabling, the fitter only resizes at every second disable. So if there was originally 5 elements in ItemContainer, the main container would only resize on 3 and 1 elements. The same applies when resizing the elements back. Because this is now in play, this can only be fixed by forcefully moving Main Container, which invalidates Fitter's purpose.

Reproduced in 5.5.0b11, 5.5.1f1, 5.6.0b7
Can't test in 5.5.0b10 and older as most of Rect Transform settings are disabled when using such setup, resulting in broken layout