With the Vex having its defalt texture changed, the Programmer's Art resource pack uses this new texture, instead of the old, pre-1.14 texture.
How to reproduce:
Set resouce pack to Programmer Art
/summon vex ~1 ~ ~ {NoAI:1}
MC-257822
Programmer Art Resouce Pack does not use old Vex Texture
Resolved
Won't Fix
0
The entire model is replaced, the old texture is not compatible with this model. Fixing this requires resource packs to be able to modify entity models.
Should be Won't Fix per the linked report.