file_exists can't find filename including ampersand on Linux

Question

I have two files: one called AB.txt, the other called A&B.txt. For each, I run the following PHP code:

echo "[$arq]";
if (file_exists($arq)) {
    echo '[found]';
} else {
    echo '[none]';
}

The outputs are [AB.txt] [found] and [A&B.txt] [none].

I tried replacing the & with \& inside file_exists(), but it also doesn't work. And it doesn't seem to be a (lack of) UTF-8 problem.

Why is that? I'm on a Linux (Debian 11) machine, using PHP 7.4.

EDIT

var_dump(scandir('.'));

Returns hundreds of files (they're on a subdirectory, actually, but let's pretend they're not). I've removed all the other files from the list bellow.

array(352) { [302]=> string(7) "A&B.txt" [303]=> string(6) "AB.txt" }

EDIT 2

var_dump($f = "diffA&B.txt", file_exists($f), touch($f), file_exists($f));

string(11) "diffA&B.txt" bool(false) bool(true) bool(true)