If you get a spawn egg and try and remove it in the creative inventory it doesn't always work. If you try and click on a different spawn egg rather than removing it will stack it. I have a gif below to show exactly what is happening. This did not used to happen and it is now, it seems to be ignoring the NBT tags and thinks it's the same item. This doesn't appear to happen with other items like wool, only spawn eggs.

Gif of what is happening:
https://gyazo.com/b32e0129343d7576c90143a4d617d999
As you can see, it stacks the item regardless of NBT tag, only removes when you click another item such as a book and quill.