Hello! Thanks for using Plain Pixel! For Arabic, Arabic, Arabic Supplement, Arabic Extended A & B, and Arabic Presentation forms A & B (minus a few ligatures) are included. If you're including anything outside of these blocks, it's possible that they might not show up. I'm a bit confused, because the "not found" glyph also isn't rendering correctly, so it's hard to know exactly where the problem may lie.
If you're willing to help, I'd love to try to solve this issue. Could you please do two things for me?
1. Please install the font on your computer, and then try putting this text in some software that supports OTF features. On Mac, this is most software. On Windows, design software like Figma, and LibreOffice should work. This will help me to know if it's a Game Maker issue, or something else.
2. Please send me any text samples where you have these blocks. You can paste them here. I'll then try them out myself and see what problems I can find, and see what I can fix. This might take me a little bit of time.
One other thing, I notice that some of the characters that are present on screen don't seem to be shaped correctly. For example, I think the first word is heart but it's hard to tell where the base characters end and the dots begin. This could be due to the size setting, or maybe the shadow. The font should display these more clearly. Please try adjusting those, and existing characters may become clearer.
Thank you for reporting this! Looking forward to your reply.