I just got a similar issue:
1. Description
Flags occasionally appear as one of an invisible and unselectable item when issued
2. Reproduction steps
Complete the second step of the "French criusers" combat misssion (this may occur with other flags, this is just the one that I got)
3. Result
An invisible and unselectable item
4. Expected result
A normal flag reward - like the one said on the description
5. Technical details
15:15 BST 11/08/2018
(image lost, sorry)