When you click play on the minecraft launcher it automatically closes and launches the game as expected, except the launcher is still in your background processes even when you close the game. When you try to re-open the launcher it won't open until you end the background processes.
Screenshot:
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As you can see the minecraft launcher is in the background but it's not open.
NOTE: The bug does not always happen, but it happens way too much.
Hey!
I have experienced this problem before as well. It seemed to be more of a problem with my computer than with the Minecraft program. I found restarting my computer fixed the issue.
You may have already tried this but if not it may help you.
Best of luck,
-Acmon