The bug

Glowing armor stands with Marker:1b do not show the glowing effect on items being displayed on the stand despite the items being visible to the player. This did not happen in 1.14.4, where marker stands displayed their items as glowing.

Additionally, even when there is no item showing, the glowing effect was still active in 1.14.4 which can be verified by modifying the glowing shader. However, now there is no glow active at all.

To reproduce

1. /summon armor_stand ~ ~ ~ {Glowing:1b,Marker:1b,ArmorItems:[{},{},{},{id:"stone",Count:1b}]}

2. Look at the armor stand and see that the item does not glow.
3. Run

   /data merge entity @e[type=minecraft:armor_stand,distance=..5,limit=1] {Marker:0b}

4. Look at the armor stand and see that the item now glows along with the stand.