Despite the fact that Armor Stands got Marker:1, they are rendering black because part of their hitbox (the top) is inside a solid block.

Steps to reproduce
1. Place a commandblock
2. Stack 3 solid blocks on top of the CommandBlock, e.g. smoothstone
3. Insert the following command into the CommandBlock and trigger it:

/summon armor_stand ~ ~2 ~ {CustomNameVisible:1b,Glowing:1b,ShowArms:1b,Marker:1b,Invisible:0b,NoBasePlate:0b,HandItems:[{id:"minecraft:lapis_block",Count:1b},{id:"minecraft:lapis_ore",Count:1b}],ArmorItems:[{id:"minecraft:iron_boots",Count:1b},{id:"minecraft:iron_leggings",Count:1b},{id:"minecraft:chainmail_chestplate",Count:1b},{id:"minecraft:player_head",Count:1b,tag:{SkullOwner:{Id:"994924dc-8404-4af4-a58b-678426ee0095",Properties:{textures:[{Value:"eyJ0ZXh0dXJlcyI6eyJTS0lOIjp7InVybCI6Imh0dHA6Ly90ZXh0dXJlcy5taW5lY3JhZnQubmV0L3RleHR1cmUvYzBmZmE0OWIxYWQ3ZmNmZDJhNWUxYTU4YzMxZjFkM2I5OTYwMDZjMTAzODBhNTQ0MTlkZmFmM2ZiNzc4MzdjZiJ9fX0="}]}}}}],CustomName:"{\"text\":\"Meri\",\"color\":\"dark_blue\",\"bold\":\"true\",\"italic\":\"true\"}"}

4. teleport the Armor Stand gradually down, e.g. with:

/execute as @e[type=armor_stand] at @s run tp @s ~ ~-0.1 ~

At some point you'll see that the Armor Stand including its equipment goes dark.

As AS are used a lot not only outside, but also inside of blocks, and using transparent blocks instead to circumvent the AS incl. their rendered equipment going dark is not always a possible option, it'd be desirable for mapmakers to have Marker:1-AS rendering themself and their equipment "dark" fixed, while keeping the other needed fixes for AS that already were made or that are currently in the making.

But, as already pointed out, it'd be very good for mapmakers if Marker:1 would not render AS/their equipment dark if stuck inside a solid block, as more options would then be possible for us.