Welcome to open source! You’ve already got a nice example of code that produces the problem - I’d make it a full MWE (minimum working example), including the using Plots so that someone can open a REPL and just copy-paste your code, and just basically include the same thing that’s in your first post.
To open the issue, just go to the GitHub page, click the issues tab, and then “new issue.” It’s probably worth searching through the issues there to see if anyone else has reported it before hand. If they have, you can add your example if it might be helpful.
Come to think of it, I’m actually not sure of it makes more sense to open the issue on the PyPlot repo or the Plots repo it could be an issue with the backend code itself, which would mean the latter.
If it were me, I’d see if I could reproduce the issue with PyPlot alone. A quick Google search suggests that this should be possible with the underlying matplotlib library.