The bug
The angry wolf texture used to look different compared to the normal wolf texture. However as of 1.14, the angry wolf texture, in the default textures, has become almost identical to the normal wolf texture.
To reproduce:
Summon a wolf and an angry wolf with the following commands:
/summon minecraft:wolf ~ ~ ~ {NoAI:1b,AngerTime:10000}
/summon minecraft:wolf ~ ~ ~ {NoAI:1b}
→ ❌ The texture of both the neutral and hostile wolves look almost identical.
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Can confirm.

I think it’s a texture change
Can confirm in 21w03a.
Can confirm in 1.16.5. Here are some steps to reproduce this issue:
Steps to Reproduce:
Summon a wolf and an angry wolf through using the following commands:
/summon minecraft:wolf ~ ~ ~ {NoAI:1b,AngerTime:10000}
/summon minecraft:wolf ~ ~ ~ {NoAI:1b}
→ ❌ Notice how their textures are almost identical compared to how they used to look.
Can confirm in 21w16a.
Can confirm in 1.17.
Can confirm in 1.17.1.