Poetry 1.2.0 cannot export a project with a cyclic dependency. I'm unsure if this is an exporter or a Poetry issue, or what would be involved in fixing it, as I have not dug into it myself yet.

$ pipx install poetry==1.2.0
$ git clone https://github.com/python-poetry/poetry && cd poetry
$ poetry export -f requirements.txt -o requirements.txt

Dependency walk failed at poetry (>=1.2.0b3,<2.0.0)

Ironically, that version constraint comes from this repository itself (poetry = ^1.2.0b3).