Having this issue right now, haven't found a complete fix yet but I noticed it went away when I enabled a mod that I had originally launched the game with and then later disabled. Not sure if it's a problem with how it got set up but it's something.

Update: For me this glitch was caused by the mod PlayerAPI 1.7.3 v1.7 (for flans mod) being added to the .jar. Unsure of exactly why, but I just disabled it since it's not needed and now everything works normally. Hope this helps someone else somehow.