The bug
On Fabulous, you can see transparent blocks like water, stained glass, etc., and particles, clouds behind semi-transparent armor stands that are not markers, but when it is a marker armor stand, you cannot see water, stained glass, and other transparent things behind it, even on Fabulous.
How to reproduce:
Switch to Fabulous!
/summon armor_stand ~ ~ ~ {Invisible:1b}
Summon an invisible marker armor stand near it using
/summon armor_stand ~ ~ ~ {Invisible:1b,Marker:1b}
Go into spectator mode
❌ You can see water, etc. through the non-marker armor stand, but not through the marker armor stand.
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Can confirm.
Video attached.

Can confirm in 21w43a.
Can confirm in 1.18.2.
Can confirm in 1.19.1.

This also applied to name tags when a player is crouched. I cannot confirm if this is still the case