Ghost Champions Bug?

On different game modes, every so often, I will see "Ghost" champions. They appear as transparent enemy or ally team members of the exact champion they chose and even have their summoner name above them. You can make out the silhouette of the champion. They even appear to attack. They eventually do disappear but this hasn't happened before. It started up a month or two ago. I saw someone in the EUW server had the same problem. I'm pretty sure this isn't an champion passive or ability either.

I've seen "ghosts" of (off the top of my head):

• Yasuo
• Nami
• Yorick
• Ahri

Does anyone else have this problem? Is it a bug in the game?