Apparently I missed something really simple, I configured my nunjucks config file incorrectly. Not for a little bit, for the past 3 years! I combined info from a bunch of tutorials and mashed them all together to form my gulpfile that powered my gulp-nunjucks-render configuration.
When starting out this is really helpful to get a feel for how something should work, though eventually you should learn more about the code and make adjustments that are best for your needs. I never did that, I just made my peace with the fact I couldn't use relative paths and had no idea why I couldn't do that.
Now after all these years, deciding to upgrade to Gulp 4, deal with breaking changes from upgrading, and having to relearn everything about Gulp I've done before, I notice this. I'm not upset since everything is a learning experience. I'm more disappointed something as simple as changing the url from
'src/' makes such a large difference.
I'll gladly have better control over my content and happily move my files back to more convenient places. Just another case of doing due diligence while coding. Don't forget to check in on your code every now and then.