XF 1.5 css.php output is inconsistent

Loading the main forum, regular CSS from the theme itself seems to be ok, but user group CSS/EXTRAS.css will sometimes not load. I can't reproduce the problem reliably -- originally I thought it was an https issue, but it doesn't seem to be.

It's not a caching issue either, since the problem will still occur in incognito or on a separate computer on a separate network.


Not working
Output: https://pastebin.com/YH40nJkk

Output: https://pastebin.com/tJJUH9CQ

I can immediately tell when the output is wrong because usernames aren't colored and the custom banners from EXTRAS.css don't load -- you can see them at the end of the working output (ctrl-f for Administrator).
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