brew upgrade not recognising CLT for Xcode

When upgrading brew installed apps I get failure messages telling me that the CLT for Xcode isn't installed, when indeed it is installed.

==> Upgrading imagemagick 7.0.11-0 -> 7.0.11-1 

Warning: Building python@3.9 from source:

 The bottle needs the Apple Command Line Tools to be installed.

 You can install them, if desired, with:

  xcode-select --install

brew config returns the following info:

macOS: 11.2.1-x86_64

CLT: N/A

Xcode: 12.4

Yet, I see the following too:

$ xcode-select -p

/Applications/Xcode.app/Contents/Developer

$ xcode-select -v

xcode-select version 2384.

I've also installed Python 3.9.2 directly from the source and I see the following:

$ python3 -V

Python 3.9.2

I also see the following :

$ ls -larth /opt/local/Library/Frameworks/Python.framework/Versions

total 0

lrwxr-xr-x 1 root wheel  59B 11 Sep 11:33 Current -> /opt/local/Library/Frameworks/Python.framework/Versions/3.8

drwxr-xr-x 6 root wheel  192B 11 Sep 11:33 ..

drwxr-xr-x 5 root wheel  160B 11 Sep 11:33 .

drwxr-xr-x 3 root wheel  96B 9 Dec 15:18 3.7

drwxr-xr-x 9 root wheel  288B 30 Dec 10:41 3.8

$ ls -larth /Library/Frameworks/Python.framework/Versions      

total 0

lrwxr-xr-x  1 root wheel   3B 25 Feb 10:34 Current -> 3.9

drwxrwxr-x  4 root wheel  128B 25 Feb 10:34 .

drwxrwxr-x 11 root admin  352B 25 Feb 10:34 3.9

drwxr-xr-x  6 root wheel  192B 25 Feb 10:34 ..

So, how am I to upgrade my brew installations now?