Confusing error related to include_dirs inside foreign_stubs with Dune 3.0


I have been getting this error message with Dune 3.0 when trying to build some FreeType wrapper with this dune file in my project:

File "libs/dune", line 8, characters 18-32:
8 |   (names          FreeType_stubs)
Error: File unavailable: /usr/include/freetype2/freetype
This is not a regular file (S_DIR)

I was able to fix the build by changing this

(include_dirs /usr/include/freetype2)

into this

(flags -I/usr/include/freetype2)

but I was wondering why this error message was appearing now. Nothing caught my eye in the recent changelogs that would explain this, and I find the error message itself quite unhelpful. Could this be a regression? Or maybe I am missing something?