Masked area is visible... in some instances

Hello.

I've recorded a subject up against a white background. Since this background was rather small, I've had to mask the rest out in Premiere Pro. However, when I view the video in PotPlayer, I can see the masked area clearly (and even shows up as grey although I added a white color matte behind it (I also tried with a white PNG) but when I view it in VLC, I can't see the masked area. The same thing happens when I view it on YouTube -- on my phone, I only see the white background but on my television, I see the masked area.

Can somebody explain why this happens and how I fix it?

Thanks!