Render fields doesn't work as expected when run outside of PW templates

Short description of the issue

The call to $child->render('offer_matrix'); is not resulting in rendering of a field, but rather a page - it's looking for "offer_matrix" in site/templates/, instead of "offer_matrix.php" in site/templates/fields/. This works as expected in a regular PW template, but it doesn't when run inside Tracy Debugger console (and perhaps also when PW is bootstrapped - but I did not test that at this point).

$myOffer = 11404;
$myOfferPage = $pages->get($myOffer);
// using absolute path doesn't work:
$str = wireRenderFile("/home/domainname/www/dev.mysite/site/templates/offer-index-dev.php", array("offer_index_page_id" => $myOffer));
// using relative path doesn't work either: (default is site/templates/ - according to docs)
$str = wireRenderFile("offer-index-dev.php", array("offer_index_page_id" => $myOffer));
// using relative path without .php suffix doesn't work either: (according to docs .php suffix can be omitted)
$str = wireRenderFile("offer-index-dev", array("offer_index_page_id" => $myOffer));

// $str is then used to generate a PDF and save it to a page - but the error appears above this
// fatal error msg: * Exception: Invalid output file location or specified file does not exist. on line: 518 in /home/mydomain/www/dev.mysite/wire/modules/PageRender.module

inside offer-index-dev.php, the relevant place where the error is triggered:

foreach($offer_index->children as $child) {
    echo $child->render('offer_matrix'); // that's where PW gives up and terminates
    // echo $child->render('/home/mydomain/www/dev.mysite/site/templates/fields/offer_matrix.php', $vars = ["pid" => $child->id]); // I've tried again with abs. path here
    // echo $child->render('/home/mydomain/www/dev.mysite/site/templates/fields/offer_matrix'); // ... and abs. path w/o .php suffix here
}

Expected behavior

I would expect that PW renders the field templates like it does when run from inside a "normal" PW template. Especially in the case where I use the absolute path, incl. file-suffix.

Actual behavior

Exception: Invalid output file location or specified file does not exist. on line: 518 in /home/mydomain/www/dev.mysite/wire/modules/PageRender.module

Steps to reproduce the issue

Create field templates, as described here https://processwire.com/blog/posts/processwire-3.0.7-expands-field-rendering-page-path-history-and-more/#field-rendering-with-template-files
Create a page that uses these fields.
In the Tracy Debugger console, use some similar code as posted above in the short description.
Try to get a value back when you run wireRenderFile()
Use the same code to output fields inside a regular PW template - you should see it there working just fine.

Setup/Environment

ProcessWire: 3.0.147
PHP: 7.2.27
Webserver: Apache
MySQL: 10.1.44-MariaDB

processwire / processwire-issues