PNG Watermarks on PNG image becomes black in transparent part

watermarked image: https://www.bingotraining.com/wp-content/uploads/2017/07/rl12-1024×645.png

source image: https://www.bingotraining.com/wp-content/uploads/2017/07/rl12.png
watermark: https://www.bingotraining.com/wp-content/uploads/2017/08/Bingo-Logo-Final-Slogan-500-White.png
options: a:5:{s:15:"watermark_image";a:16:{s:10:"plugin_off";b:1;s:19:"manual_watermarking";b:1;s:9:"extension";s:0:"";s:15:"frontend_active";b:0;s:19:"deactivation_delete";b:0;s:8:"position";s:13:"middle_center";s:12:"offset_width";i:0;s:13:"offset_height";i:0;s:3:"url";i:4018;s:19:"watermark_size_type";i:2;s:14:"absolute_width";i:0;s:15:"absolute_height";i:0;s:5:"width";i:28;s:11:"transparent";i:25;s:7:"quality";i:90;s:11:"jpeg_format";s:8:"baseline";}s:12:"watermark_on";a:3:{s:5:"large";i:1;s:6:"medium";i:1;s:12:"medium_large";i:1;}s:16:"watermark_cpt_on";a:3:{s:18:"question_type_desc";i:1;s:3:"tip";i:1;s:8:"exercise";i:1;}s:16:"image_protection";a:3:{s:10:"rightclick";b:1;s:11:"draganddrop";b:1;s:9:"forlogged";b:1;}s:6:"backup";a:2:{s:12:"backup_image";b:1;s:14:"backup_quality";i:90;}}