concatMapM has multiple problems. For one, it has a rather confusing name for a function defined like this:

concatMapM
    :: ( Applicative f
       , Monoid m
       , Container (l m)
       , Element (l m) ~ m
       , Traversable l
       )
    => (a -> f m) -> l a -> f m
concatMapM f = fmap fold . traverse f

There is nothing related to M here. Moreover, the u

For some reason, the debugger starts working in the created projects when we remove these lines:

[tool:pytest]
addopts = --cov=project/