GLib2 tries to find "python3" and if it can't find it, it will go for "python"; if port select wasn't explicitly run, this will likely end-up with Python 2.7. As a fallback, meson can use whatever python it's running on if the argument to find_installation is empty.
--- meson.build.orig	2026-02-10 12:21:31
+++ meson.build	2026-02-10 12:25:27
@@ -2506,13 +2506,13 @@
 
 glib_conf.set('HAVE_PROC_SELF_CMDLINE', have_proc_self_cmdline)
 
-python = import('python').find_installation()
+python = import('python').find_installation('')
 
 # Sets the shebang on script files. This has been changed many times including
 # using env, the absolute path to the Python executable, and using both python
 # and python3 names. Please review the history here, prior to suggesting
 # further changes: https://gitlab.gnome.org/GNOME/glib/-/merge_requests/4391
-python_shebang = '/usr/bin/env python3'
+python_shebang = '@PYTHON@'
 
 python_version = python.language_version()
 python_version_req = '>=3.7'