UPDATE: GM is now aware of this bug. The fix is public information but also below in the second post.

To reproduce:
1. Make sure there is some percentage on Season 4
2. Transfer emblem from season 4 to 5
3. Add more ingredients until 100%
4. Cannot enhance (it is grey)

You can enhance below 100% which is a bad idea.

See the image:

The fix is to fill up the ingredients to 100% and leave. Come back and it will let you enhance.