YZ250F (2018) starting issue

Hello,

I was riding on Saturday and suddenly my engine quit and I couldn't get the bike restarted. This happened during my first lap. As I was trying to get it started, I noticed pressure building in the kick starter. It got to the point where it wouldn't move. Someone looked at it with me. He put the bike in gear and turned the rear tire. This released the pressure and the bike started normally. It sounded fine at idle and when I turned the throttle. Also, no signs of power loss or bogging sound. I went back out on the track and the same thing happened again. I can push the bike without issue, so I don't think the transmission has seized. Spinning the tire on the stand, everything sounds normal.

When I started the bike for the first time on Saturday, it started on the third kick like it has since I bought it. It ran fine and didn't show any signs of quitting. Any idea what may be going on? I'm really hoping it is not valve or piston related. It doesn't seem like it would be if I was able to get the bike restarted and it runs like it always has.

One thing I should mention is I washed the bike Thursday before riding. Afterwards, I started it, but forgot to remove the exhaust plug...doh! It started, but for less than 5 seconds. I then removed the plug. When I removed the plug, I heard air or something come out of the exhaust. Could this be related to my problem?

Thanks in advance!!