Creating a file with a parseable extension then changing it keeps the previous extension's parsing

If I create a file like 'someFile.php', which has some PHP or HTML or whatever in it, which colors the syntax and runs validation on it, then change the name to 'someFile.txt', it continues to treat the file as if it's a .php file.

I can't tell if this is a bug, but I'm not seeing how to fix the formatting to match the new extension.

If I delete the file then recreate it with .txt, it's treated properly as a text file.

I can't make sense of this logic, and it's counter-intuitive, so it seems to me to be at least a design flaw.