I don’t think this particular behaviour is documented yet. If it is, it wasn’t visible enough for me to find it: I as well struggled with this issue a few months ago.
The confusion
In particular, I had to look through Julia’s code to understand that any entry in LOAD_PATH, whose directory contains a Project.toml file, will be treated as a “project environment” only. In other words: having a Project.toml file inhibits Julia from using it as a “package repository”.
An introduction, for now
At the moment, I would suggest you look at some of the posts I have written, starting with the “tips” found here:
→ Proper way of organizing code into subpackages - #3 by MA_Laforge
I also suggest you read the two posts of mine that directly follow it. I tried my best to give useful insight.
You might also be interested to take a look at the improved package/module documentation (v1.7):
→ Modules · The Julia Language
Overall though, I think the package/module documentation still needs some improvement (I just haven’t figured out how, exactly).