WHATSAPP users have been left baffled by a mysterious bug that appears to be blacking out flag emojis.

Twitter is being flooded with confused WhatsAppers who say the flag won't show up correctly in the popular messaging app.

One user tweeted: "When I type 'England' on WhatsApp, the George's Cross flag comes up above keyboard as it should appear..."

They continued: "But when I send, it's coming up as a black flag."

The bewildered app user shared an image of the issue, showcasing the mysterious black flag emojis.

WhatsApp provides an official black flag emoji that looks exactly the same as the one in the user's image, so it's not clear if this is simply a bug or hoax.

Another Twitter user called PinkPrincess posted: "So why the hell has the Scotland flag on WhatsApp turned Black?"

In her screenshot, there's a message that clearly depicts a black flag, although it's impossible to tell whether the emoji was originally inputted as a Scottish flag.

The Deputy Head of news at the Guardian US experienced similar issues with the Scottish flag, posting: "Has anyone else tried using the Scottish flag emoji in WhatsApp lately?"

He proceeded to tweet an image showcasing the same black flag from previous tweets.

We tested bug here at The Sun offices, and found it impossible to replicate on both Android and iPhone devices.

Interestingly, it appears that the issue is only affecting certain flags, with one tweet showcasing an example of the England flag blacking out – while other flags still appear:

This issue doesn't appear to be new either; Twitter use @ula_trochimiak posted a video of the bug in action, way back on December 14, 2017.

The clip clearly shows a Scottish flag being correctly inputted into the chat box, only to be swapped out for an all-black version once it's sent:

According to Twitter posts, it seems like English, Scottish, and Welsh flags are most commonly affected.

The earliest post reporting the issue appears to be back on November 17. The user says he tried to post a Scottish flag on an Apple device running iOS 11.1.1.

WhatsApp doesn't appear to have explained the issue publicly, and it's not clear at this stage whether the Facebook-owned chat app is even aware of the problem.

The Sun has contacted WhatsApp for clarification on why this bug might be happening, and will update this article with any response.