You can't see mobs if you watch through glass pane that is held by other mob. To see it just summon some zombies with egg, and then summon giant with white glasspane:
/summon Giant ~ ~ ~ {HandItems:[0:{id:"minecraft:stained_glass_pane",Count:1b,Damage:1s}]}
Notice that some of the zombies are seen through glass in attached screenshot
bug searched by tags: through glass pane
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behind there are many of them. Every column has 2 zombies holding something. This is map shared by Searge Grum via twitter, as I remember.
It is not consistant bug, but it is still a bug.
"Grum's Army" : https://twitter.com/_grum/status/627923649828360193
Confirmed.