Weird caching behaviour when reloading pages

When reloading pages on my server, they alternate between the current version and either an older version or a 404 error. I’ve tested this on multiple browsers.

You can check here:
eg. https://www.promostuff.com.au/admin/test01.php

And this is what I get in developer tools when the 404 error is generated:

I’m using PHP / Apache.

When testing on my localhost the page always loads normally. Has anyone got an understanding of what might be happening?

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