did /summon Rabbit ~ ~ ~

to get a Killer Rabbit of Caerbannog and tried to rename it "Toast" to get a Killer Toast Rabbit

When renamed, It did NOT take the name of "Toast" but instead took the name of "The Killer Rabbit of Caerbannog" as seen in the provided screenshot.

To replicate this

1. do /summon Rabbit ~ ~ ~

(Or naturally get it)
2. rename it with a name tag (Ex:"Toast","Bob",etc.)
3. watch it not change but get the name of "The Killer Rabbit of Caerbannog", even though that is not what you wrote on the name tag.