Noticed that:

# models/in_process/sysconfig_get_paths.py # L31
def get_paths(kind="default", vars=None):
    ...
    else:
        if sys.platform == "darwin" and "osx_framework_library" in scheme_names and kind == "prefix":
            return sysconfig.get_paths("posix_prefix", vars=vars)
        return sysconfig.get_paths(vars=vars)

ubuntu's sys.platform is 'linux', kind is 'prefix', so it'll run sysconfig.get_paths(vars=vars), which would use default scheme "posix_local", but when loding plugins,

# core.py # L314
    def _add_project_plugins_library(self) -> None:
        ...
        scheme = "nt" if os.name == "nt" else "posix_prefix"
        purelib = sysconfig.get_path("purelib", scheme, replace_vars)

the scheme is "posix_prefix", that's the reason that installation and loding routes are different. I tried to change scheme to "posix_local", and the plugins could be loaded successfully.

Since I only have windows and ubuntu system computers, I'm not sure how to fix it correctly in all systems. On my ubuntu computer, sysconfig.get_scheme_names() got ('deb_system', 'nt', 'nt_user', 'osx_framework_user', 'posix_home', 'posix_local', 'posix_prefix', 'posix_user'), no osx_framework_library. So maybe it should be changed to

        if kind == "prefix":
            if (sys.platform == "darwin" and "osx_framework_library" in scheme_names) or (sys.platform == "linux"):
                return sysconfig.get_paths("posix_prefix", vars=vars)

? But I'm afraid it may fail on other systems such as centos

Perhaps

if kind == "prefix":
    if (sys.platform == "darwin" and "osx_framework_library" in scheme_names) or (sys.platform == "linux" and "deb_system" in scheme names):

would be better, but I have no way to test it on other systems

Here's the sysconfig from a macOS 14.7 (Apple silicon and Python 3.11.10):

('nt', 'nt_user', 'nt_venv', 'osx_framework_library', 'osx_framework_user', 'posix_home', 'posix_prefix', 'posix_user', 'posix_venv', 'venv')

Here's the sysconfig from a macOS 14.7

I believe pdm is adapting macOS well because it checks if the platform is "darwin". But it dosen't check for "linux". What I'm afraid is linux systems other than debian and derivatives, I don't know their result of syscomfig.get_default_scheme(). But as load_plugins use "posix_prefix" for all systems besides os.name == nt, I doubt if

if kind == "prefix" and os.name != "nt":
    return sysconfig.get_paths("posix_prefix", vars=vars)

is enough.