Discovered during testing
MDL-39526, see the matching test instructions.
From step 10.
Browser 1 (admin) the first upgrade page is now displayed
Browser 2 (student) refresh course page => no warnings anymore
Browse 1 click on continue button
Browser 2 refresh the page multiple time. All good, toward the end the CSS disappears.
Browser 1 is on the server checks page confirmation (CSS disappear here too)
Browser 1 click on continue button
Browser 2 refresh, very quickly the CSS issue disappears, all go back to normal.
Browser 1 display correctly the plugin confirmation page
Browser 1 clicks on Upgrade database button
Browser 2 refresh and see the course page without CSS and with this warning:
Note I wrote an issue for the warning: