fallback sprite assets are not being used in textcore

Fallback assets are ignored even when the missing sprite can be found in the fallback sprite asset, to reproduce:

1. open attached project
2. Note that EmojiOne  is being used as default sprite asset in UITK Text Settings/EmojiOne has a fallback asset called EmojiOne 1
3. Also not that EmojiOne 1 has an additional glyph with index =15
4. create a label in ui builder and use "<sprite=15>" as the text string

Expected result: a smiley face is displayed at runtime

Actual result: sprite doesn't display

I have tested the tmp/textcore sync branch as well, issue persists. It appears the GetSpriteCharacterFromSpriteAsset_Internal is never called? (if I looked at the right place) 

NOTE: This issue does not occur when using name attribute such as <sprite name="sprite_name">