Are you playing a champion that is reliant on approaching their enemies unseen? Have you been finding that hard to do this preseason? This is why, and it's not getting fixed any time soon.
There are two types of stealth in League of Legends. The first is camouflage, which only makes a champion invisible under certain conditions, and at range. If you get close enough, you will be spotted by enemy champions without the use of a pink ward of red trinket.
This is used by champions such as Akshan, Pyke, and Twitch.
Invisible champions can only be revealed by True Sight and will briefly shimmer whenever they take damage. A unit will shimmer for every instance of damage they take, and shimmering doesn't make a unit targetable nor remove the stealth effect, but grants positional information to the enemy.
This mechanic has been bugged since the start of the preseason, with both camouflage and invisibility randomly not working due to changes to the map, or items used.
The fact that stealth, a mechanic shared by upwards of 15 champions has not been fixed for the entire duration of preseason is worrying. Especially so when the preseason in question has had minimal updates to the game overall.
As it stands, and with new bugs coming along with new patches, Riot are not making a move on fixing this issue, and stealth-based champions will be crippled for quite some time yet.