The bug
Item frames with an item inside them appears to be blank when the player views it through glass (and possibly other transparent blocks). When the glass is removed it displays the item as usual.
Related issues
relates to
Attachments
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More information: only happens with items that are using generic item model (non 3d model) and looking through any semi-transparent block. tested with multiple colors of stained glass, honey block, and slime block. tested multiple with blocks and items in item frame.
Relates to MC-165225 I guess.

spotted the same problem. But funnily redstone dust shows up. (All other brewing ingredients did not)
[media]
Fixed in 1.15 pre-1.

Confirmed fixed 1.15 pre-1.