Environment variable in .prj file

[Simon Wright]

$HOME is /Users/simon. In /Users/simon/coldframe/examples, I have a Wisi 
project file Simple_Buttons_Scripting.prj:

gpr_file=Simple_Buttons_Scripting.gpr
gpr_project_path=..
gpr_project_path=$HOME/scripted_testing
gpr_project_path=$HOME/tcladashell
$BUILD=Debug

I'm 99.9% sure it used to work, but now the resulting project file search path 
is

/Users/simon/coldframe
/Users/simon/coldframe/examples/$HOME/scripted_testing
/Users/simon/coldframe/examples/$HOME/tcladashell

whereas I was expecting/hoping it'd be

/Users/simon/coldframe
/Users/simon/scripted_testing
/Users/simon/tcladashell

I tried using ${HOME}, no difference (except that the search path now shows the 
{}).

This is with 7.1.3 from git, commit 30b8ea0d3d673b50e8feeabbde7355d87b87c923 of 
Thu Jun 4 15:19:17 2020 -0700

I'm sure I've missed something, but is there a way of completely wiping out all 
traces of current projects? At the moment I have to stop/restart emacs

LATER

I made this experimental change

======================================
diff --git a/gnat-core.el b/gnat-core.el
index 5901b223..be008baa 100644
--- a/gnat-core.el
+++ b/gnat-core.el
@@ -86,8 +86,10 @@ Throw an error if current project does not have a 
gnat-compiler."
   ;; We maintain two project values for this;
   ;; project-path - a list of directories, for elisp find file
   ;; GPR_PROJECT_PATH in environment, for gnat-run
-  (let ((process-environment (copy-sequence (wisi-prj-file-env project))))
-    (cl-pushnew dir (gnat-compiler-project-path compiler) :test #'string-equal)
+  (let ((process-environment (copy-sequence (wisi-prj-file-env project)))
+        (d (substitute-in-file-name dir)))
+    (message "adding project dir %s (was %s)" d dir)
+    (cl-pushnew d (gnat-compiler-project-path compiler) :test #'string-equal)
 
     (setenv "GPR_PROJECT_PATH"
            (mapconcat 'identity
======================================

but the messages are

adding project dir /Users/simon/coldframe/examples/$HOME/tcladashell (was 
/Users/simon/coldframe/examples/$HOME/tcladashell)
adding project dir /Users/simon/coldframe/examples/$HOME/scripted_testing (was 
/Users/simon/coldframe/examples/$HOME/scripted_testing)
adding project dir /Users/simon/coldframe (was /Users/simon/coldframe)

so something wrong with environment vars?