Yep, it's because after formatting, the partition gets wrong name and type code.
Use a small utility, called "gdisk" (GPT Fdisk), install it and run it from the terminal.
- the "p" switch will print to you various details
- the "v" switch will verify the disk and print out errors, if found

You will see that neither the name of the EFI-partition is "EFI", nor
the type code is "ef00", which is the proper one for a EFI System Partition.

And for the correction, you will need two switches :
- "c" for changing the partitions name
- "t" for changing the type code to "ef00"
- "w" for writing the changes to the disk

Unmount the partition, restart and you'll not see it mounted anymore
Hope this will help you out.

Greetings

P.S. It seems that i have overlooked a user's comment, which already helped