You lost because your not was trash mate. John and Thresh gave kills to that Xayah who's fed as all hell. Karma got assists from her and as such was strong with her shields.
In other words, you didn't damn your team, the bot lane did. As such, no LP should be given back
On to the bug. First of all, you should have never engaged into a fight if you didn't see health. That's like running into a minefield and not knowing where the bombs are. It's just a recipe for disaster.
For starters, DC and come back. If it doesn't work, reboot the client and enter into the game. If that's fails, it's a very serious bug and you should not only contact support, but start to repair your client. While submitting that ticket, pick up and download the Hextech Repair Tool. Run it, and test to see if you have a problem in a Practice or Custom game. If it still persists, initiate and full repair in the client options. If that's also not working, support will get to you and you can fix it with them.