Smarty v5 built-in json_encode modifier doesn't take Smarty $_CHARSET encoding into account, and can't be overridden using registerPlugin()

I manage a couple of websites, some of which use cp1252 encoding.
It's in these websites, the json_encode modifier sometimes returns false when given cp1252 encoded data.
This used to work with Smarty v3 and v4 where I registered my own json_encode modifier that does the input encoding conversion automatically based on the Smarty $_CHARSET encoding.
After some debugging, I noticed that my custom and registered modifier isn't being called at all in v5, but instead the built-in Smarty json_encode() modifier is being used.

I've tried to unregister the built-in json_encode modifier first before registering my custom modifier but that doesn't seem to have any effect:

$smarty->unregisterPlugin('modifier',  'json_encode');
$smarty->unregisterPlugin('function', 'json_encode');
$smarty->registerPlugin('modifier', 'json_encode', [$this, 'modifier_json_encode']);

If the json_encode modifier is to ignore the $_CHARSET encoding by design, then how do I override a built-in modifier/function with a customized one in v5?

Start by reproducing the cp1252 input failure with Smarty v5's built-in json_encode modifier and the registerPlugin/unregisterPlugin calls shown in the issue. Trace the v5 modifier registration and lookup path to determine why the custom modifier is bypassed; done means charset handling and overriding behavior are defined and covered by a regression test.