My guess is that the reflection added is off once we have extended Table classes in between.
I will try to find a way to clarify/reproduce.

I found the issue:
In RulesProvider you slice it off again, commenting it out makes it also work:

    if ($argument->getName() !== 'context') {
        //$arguments = array_slice($arguments, 0, -1);
    }

I guess that happens because the new code by reflection checks already only adds the context conditionally:

    if ($lastParam && $lastParam->getName() === 'context') {
        $args['context'] = $context;
    }

    $result = $callable(...$args);

So in certain cases it gets not added, but then "stripped" even though it was never there.

In RulesProvider you slice it off again, commenting it out makes it also work:

That's done in reference to the issue you submitted #17631.

Yeah, that whole context thing being randomly at positions is bonkers in such an API.
Quite problematic.

In terms of BC, I wonder how we can fix things up here.

Ah, and my application is using

namespace App\Validation;

use Cake\Validation\Validation as CoreValidation;
use PhpCollective\DecimalObject\Decimal;

class Validation extends CoreValidation {

	/**
	 * @inheritDoc
	 */
	public static function decimal(mixed $check, int|bool|null $places = null, ?string $regex = null): bool {
		if ($check instanceof Decimal) {
			$check = (string)$check;
		}

		return parent::decimal($check, $places, $regex);
	}

}

So I guess this could be the issue how to reproduce it.

OK, now that I remember the "context" of the ticket/issue:
Remember how we found a workaround?
dereuromark/cakephp-decimal@246a6c8

Well, turns out with the updates around reflection we must remove that workaround.

So removing

// in your bootstrap
\Cake\Validation\Validator::addDefaultProvider('default', new \Cake\Validation\RulesProvider(\App\Validation\Validation::class));

makes it work

Do we need to fix then something? As this could have other people run into the same trap if they modified this.
Or do we just ignore it as "works-for-me" ?

I have already poked at the validator classes multiple times and each time fixing one issue seems to create another one. So I not looking forward to poking it again.

We can merge this and maybe @markstory or someone wants to comment on the open ticket regarding the strategy moving forward. Then we can close this topic now that we know how to solve it.