I've been working with Magento for a number of years but I've never come across this before. I'm working on a custom theme for a new site - vanilla Magento 2.4.3 Open Source install - and recently after adding some custom CSS and changing the minicart icon, I keep getting 403 errors from resources in the /pub/static directory. I've set the correct permissions, clean cache, flush cache and the page (any page) will load fine once. If I navigate to another page, cart, anything, the next page that loads returns 403's all from the /pub/static directory.
I know it's not my theme - the admin area is behaving the same way. I managed to set the theme back to Luma and the problem persists.
My dev server is a bare metal machine running Ubuntu 20.04 with Apache, MariaDB, Elasticsearch and Redis.
I've only found [this]New Magento 2.4.1 installation with assets only appearing on first page load, when not created yet in static folder other thread with an issue similar but it's not quite the same.