HTML entities not generated properly in parsedown extensions

[toodlelew]

Environment: Parsedown 1.7.4 in PHP 5.6.40.

Learning how to use Parsedown in order to use it into a PHP-based recipe program to manage recipe instructions.
I've used the example extension ("Add Inline Element") as a basis for a test to handle a markdown extension that will manage measurement unit conversions ("pounds" to "kilograms", "degrees Centigrade" to "degrees Fahrenheit", etc.). The initial test is to recognize a special markdown that indicates temperature in Fahrenheit, and expresses it as "° F".

The extension seems well-formed, properly recognizing the markdown, and (mostly) generating the expected HTML. However,
it does not properly generate the HTML for the ° entity,

Given the input
"Preheat the oven to {F}350{/F} and wait 10 minutes."
I expect the output HTML to be
<p>Preheat the oven to <em>350&deg; F</em> and wait 10 minutes.</p>

However, the invocation of the extension results in
<p>Preheat the oven to <em>350&amp;deg; F</em> and wait 10 minutes.</p>

See attached test php program

testparsedown.txt

It is worth noting that, If I enter the input
"Preheat the oven to 350° F and wait 10 minutes."
directly into parsedown, I get the expected HTML output; parsedown does not convert the input ampersand to an entity

Note: 2024-03-01 edits to clarify expected, actual results.

[toodlelew]

Expanded test case, and include expected & actual results

testparsedown.txt

exptected_results.txt

actual_results.txt