wpautop adds an extra <p>

[bobbingwide]

With the following input

$input      = "<div>\n<div></div>\n" .         "<span></span></div>\n";

wpautop produces the unexpected output

$unexpected = "<div>\n<div></div>\n" . "<p>" . "<span></span></div>\n";

This is very similar to #39377.
Note: I added the newlines to the input to make the difference more obvious.

• div before span gives p
• span before div gives /p

Workaround

For my affected plugin I've written a PHPUnit test case that will fail when wpautop's behaviour no longer produces the unexpected results.
And I'm changing my code to avoid mixing spans and divs.

[pento]

Duplicate of #27350.