smarty v4.5.5 with force_compile=true and deprecation notices

I was updating smarty v4.3.4 to v4.5.5 and this deprecation notice started popping up:

Deprecated: Using unregistered function "strpos" in a template is deprecated and will be removed in a future release. Use Smarty::registerPlugin to explicitly register a custom modifier. in smarty4/sysplugins/smarty_internal_templatecompilerbase.php on line 665

When updating from v3 to v4, I've created bunch of modifier functions in custom plugins folder, eg
myplugins/modifier.strpos.php:

function smarty_modifier_strpos($haystack, $needle, $offset = 0)
{
    return (strpos($haystack, $needle, $offset));
}

I registered myplugins folder with addPluginsDir and everything worked.

Now on v4.5.5, if $smarty->force_compile = true; then notices pop up.

They go away if I:

a) set $smarty->force_compile = false

or

b) register manually those modifiers like this

 $smarty->registerPlugin('modifier', 'strpos', 'strpos');

I tried to wrap my head around https://github.com/smarty-php/smarty/issues/813 and https://github.com/smarty-php/smarty/discussions/967 but cannot find what is "new proper way" of dealing with custom modifiers.

Why are custom modifier ignored if force_compile is set to true?

Reproduce the notice with Smarty 4.5.5, force_compile enabled, and the custom modifier setup described in the issue. Start at smarty_internal_templatecompilerbase.php line 665 and compare plugin discovery through addPluginsDir with explicit registerPlugin calls; use issues #813 and discussion #967 for the intended behavior. Done means explaining or correcting why the modifier is ignored during forced compilation.