Affects Version/s: Minecraft 1.8.3, Minecraft 1.8.8, Minecraft 15w36c, Minecraft 15w47c, Minecraft 16w38a, Minecraft 1.11.2, Minecraft 17w06a, Minecraft 1.12, Minecraft 1.12.1, Minecraft 1.12.2 Pre-Release 1, Minecraft 1.12.2 Pre-Release 2, Minecraft 1.12.2, Minecraft 17w43a, Minecraft 17w43b, Minecraft 17w45b, Minecraft 17w46a, Minecraft 17w47a, Minecraft 17w47b, Minecraft 17w48a, Minecraft 18w01a, Minecraft 18w03b, Minecraft 18w16a, Minecraft 18w19b, Minecraft 18w21a, Minecraft 18w21b, Minecraft 18w22a, Minecraft 1.13-pre1, Minecraft 1.13-pre2, Minecraft 1.13-pre3, Minecraft 1.13-pre4, Minecraft 1.13-pre6, Minecraft 1.13-pre7, Minecraft 1.13-pre8, Minecraft 1.13-pre9, Minecraft 1.13-pre10, Minecraft 1.13, Minecraft 18w30a, Minecraft 18w30b, Minecraft 18w31a, Minecraft 18w32a, Minecraft 18w33a, Minecraft 1.13.1-pre1, Minecraft 1.13.1, Minecraft 1.13.2-pre1, Minecraft 1.13.2-pre2, Minecraft 1.13.2, Minecraft 18w47b, Minecraft 18w48a, Minecraft 18w48b, Minecraft 18w49a, Minecraft 18w50a, Minecraft 19w02a, Minecraft 19w03b, Minecraft 19w03c, Minecraft 19w04b, Minecraft 19w05a, Minecraft 19w06a, Minecraft 19w07a, Minecraft 19w12b, Minecraft 19w13b, Minecraft 19w14a, Minecraft 19w14b, 1.15.2 Pre-Release 1, 1.15.2, 20w06a, 20w09a, 20w12a, 20w13a, 20w13b, 20w14a, 20w17a, 20w19a, 1.16 Pre-release 5, 1.16.1, 1.16.2 Pre-release 1, 1.16.2 Release Candidate 1, 1.16.4, 20w46a, 20w48a, 20w49a, 20w51a, 21w03a, 1.16.5, 21w05b, 21w06a, 21w07a, 21w11a, 21w18a, 1.17 Pre-release 3, 1.17, 1.17.1
Fix Version/s: None
Since at least 21w20a, Marker-true-armor stands don't show bubbles if submerged into water as described below, in case there is a block below it. It doesn't matter if it's a full opaque or transparent block, and not only glass, but also e.g. slabs or a banner works.
I don't know if this is WaI, I couldn't find anything in the Wiki's armor stand history, but the as-is status could be a potential workaround.
Note to map/contraption makers: If you need the space below a water-submerged, invisible Marker-true-AS to be a nonobstructive void, a barrier block would of course not be an option, but you can use e.g. a structure void block (or e.g. the newish light block). You'll surely come up with better ideas as well.
I don't know if it's WaI that Marker-false-AS still create bubbles if fully submerged in water as described below, and if the above workaround for Marker-true-AS is sufficient for mapmakers, or if they'd prefer a full fix.
I hope Mojang can ask the mapmaking community for their requirements, and, dependent on that, this bugpost may be closed.
As long as this is not officially decided, I'll continue to update this bugpost.
Leaving the old bugpost text below for history and comprehension purposes.
Armor stands with Marker set to true create bubbles underwater which breaks immersion, as those marker-armor stands are only usable/producible in Creative and valuable mapmaker tools. Armor stands get this behaviour as they are part of EntityLiving. As soon as the armor stand's "eye height" is underneath water, bubbles appear after 300 ticks (15 seconds).
That's why an armor stand with Marker false does not create bubbles, as long as it's solely with its "feet" in water, but not with its "head" or rather at its eye height:
As soon as a Marker-false armor stand is underwater at eye height, it'll also create bubbles like the Marker-true armor stand:
As the Marker true armor stand got its tiny hitbox only at its "feet", bubbles appear in any case, when standing in water, after 300 ticks.
An easy way to solve this without ugly special cases could be to just let canBreatheUnderwater() return true for armor stands.
Wait for 300 ticks on newly summoned armour stands for bubbles to appear.