When creating a text display and attemping to change its transformation with interpolation enabled, the entity does not interpolate, but rather instantly transforms to its new transformation.

To reproduce, simply run the following commands in the chat:

/summon minecraft:text_display ~ ~ ~ {text:'"Hi"',billboard:"center",Tags:['test']}
/data merge entity @e[type=minecraft:text_display,tag=test,limit=1] {start_interpolation:0,interpolation_duration:10,transformation:{scale:[2.0,2.0,2.0]}} 

If run on a singleplayer world or a vanilla server, the text display will interpolate correctly to the new scale amount as expected. However, if run on a fresh Spigot server with no plugins, then the text display will not interpolate and rather instantly double in size.