For a project I work on, we have a different package source configured as primary (since poetry deprecated default) with PyPI as second primary, as recommended in https://python-poetry.org/docs/repositories/ .

This is the relevant section of our pyproject.toml:

[[tool.poetry.source]]
name = "galaxyproject"
url = "https://wheels.galaxyproject.org/simple"
priority = "primary"

[[tool.poetry.source]]
name = "PyPI"
priority = "primary"

When exporting with poetry export -f requirements.txt --without-hashes --output requirements.txt the output file starts with just:

--index-url https://wheels.galaxyproject.org/simple

so when trying to install the project dependencies from this file, it fails with:

Looking in indexes: https://wheels.galaxyproject.org/simple
...
ERROR: Could not find a version that satisfies the requirement a2wsgi==1.10.4 (from versions: none)
ERROR: No matching distribution found for a2wsgi==1.10.4

Before the merge of #263 , the file started with:

--extra-index-url https://wheels.galaxyproject.org/simple

and it worked fine.
I think the proper solution is to also add for each primary after the first an --extra-index-url URL line, e.g. adding the following line fixes the issue for us:

--extra-index-url https://pypi.python.org/simple