So... I think I figured this out.
I tried a couple of things, including different rhub platforms and all was fine. Eventually, I re-cloned the package in a new folder and project, activated renv, installed packages. And... everything worked fine, from the project with renv enabled.
So I thought I'd just wipe my R folder with all packages, and install them anew. But no, I had even more errors, with other packages, besides purrr and dplyr, such as httr.
I then installed manually all these packages with install.package(), and... now all is fine.
In brief, the issue must be that packages that are installed via the OS package manager are not visible to devtools::check(). To make them available to check, I apparently need to install them again from inside R, even if otherwise everything works just fine.
(Fedora has in its repo many R packages, that I installed with something like sudo dnf install R-httr... these are useful, as they solve a number of issues with dependencies).