Watermark on image randomly gets gray artifacts - php

Question

I have the task to develop a plugin for WordPress that adds watermark to uploaded images using PHP. WP hosted on AWS Bitnami. They all need to be exactly 800x800px so I upscale/downscale the source image and then add the PNG watermark.
For some reason, most of the time images end up with terrible grey artifacts on them. 5% of the time they are perfectly okay, I could not see any patterns that lead to success.
This is what happens:
Image with failed watermark
Image with good watermark
This is my code:
function add_watermark($resized_image) {
ini_set('memory_limit', '256M');
$watermark = imagecreatefrompng( '/transparency-award-watermark.png' );
imagealphablending($watermark, false); // Saw that on StackOverflow, didn't help
imagesavealpha($watermark, true); // That one too
$info = getimagesize($resized_image);
if(max($info[0], $info[1]) != 800) {
resize_image($resized_image);
}
if( $info[2] == IMG_JPG or $info[2] == IMG_JPEG ) $image = imagecreatefromjpeg($resized_image);
if( $info[2] == IMG_PNG ) $image = imagecreatefrompng($resized_image);
$sx = imagesx($watermark);
$sy = imagesy($watermark);
imagecopy($image, $watermark, imagesx($image) - $sx, imagesy($image) - $sy, 0, 0, imagesx($watermark), imagesy($watermark));
imagejpeg($image, $resized_image, 95);
imagedestroy($image);
imagedestroy($watermark);
}
function resize_image($orig_image) {
$info = getimagesize($orig_image);
if( $info[2] == IMG_JPG or $info[2] == IMG_JPEG ) $source_image = imagecreatefromjpeg($orig_image);
if( $info[2] == IMG_PNG ) $source_image = imagecreatefrompng($orig_image);
$source_imagex = imagesx($source_image);
$source_imagey = imagesy($source_image);
$dest_imagex = 800;
$dest_imagey = 800;
$dest_image = imagecreatetruecolor($dest_imagex, $dest_imagey);
imagecopyresampled($dest_image, $source_image, 0, 0, 0, 0,
$dest_imagex, $dest_imagey, $source_imagex, $source_imagey);
imagejpeg($dest_image, $orig_image, 100);
}