As suggested in the comments, the answer here are circular dependencies.
There is actually no circular dependency present in the code provided in the question (because it’s just a simplified snippet of code) but the symptoms are very clear.
The simplest example of a circular dependency is when file A imports file B and file B imports A. Unfortunately, what makes this issue hard to detect sometimes is that the circle can be arbitrarily large, spanning over a huge amount of files.
Circular dependencies are supported in ES6 and they can be used when one is careful enough. However, my takeaway here is that circular dependencies are very often a sign of bad design decisions. Which was exactly my case.