1It turned out not to be the issue,
2but rather a "subapplication" within the main application
3was autoloading an earlier version of guzzle.
4
5So this turned out to the result of poor design
6and then having 2 composer autoloaders mixing up with eachother.
7
8Running composer update in the subapplication
9resolved the issue, as it was autoloading the functions.php
10from the older guzzle version:
11
12 - Updating guzzlehttp/guzzle (6.3.3 => 6.5.2): Loading from cache