Any time I either click on fullscreen in the video settings menu, or hit F11, the window will alway move to my main monitor and display fullscreen there.

Usually windows display in fullscreen on the monitor it's currently in.

On multiple attempts the results were the same.

Steps to Reproduce:
1. Open Minecraft
2. Can move the window to the second monitor before or after the next step, doesn't matter
3. Go to Options... --> Video Settings...
4. Click Fullscreen: OFF (to change it to Fullscreen: ON)
5. Alternative to Steps 3 & 4, just hit F11, results will be the same

Attachments should give a good idea about what's going on.
Monitor1 is 1920x1080
Monitor2 is 3840x2160