Using alpha channel with VP8 and VP9 gives strange results on different mobile devices

[The-Linguist]

Using Adobe Media Encoder, I have produced two webm files (one with VP8 encoding and another with VP9 encoding) from the same source avi file that has alpha information. I have run tests to view them on different devices. On a SONY phone (android/chrome 74) the one with VP8 encoding is displayed correctly with a transparent background while the one with VP9 encoding has its background filled with solid black instead of transparent. On a HUAWEI phone (android/chrome 74) it is the exact opposite! The one with VP9 is displayed correctly while the one with VP8 has its background filled with solid black instead of transparent. On an ASUS tablet both the VP8 and VP9 are displayed correctly with transparent backgrounds. On a XIAOMI phone neither VP8 nor VP9 works for alpha transparency. As I need to use webm with alpha in my web application, I want my files to be displayed correctly on as many devices as possible. Like the soccer player on this example: https://simpl.info/videoalpha/ What happens when I view that page on -> SONY phone; both the dancer and the soccer player are displayed correctly. HUAWEI phone; the background in the dancer's video is filled with solid gray while the soccer player's video is displayed correctly. XIAOMI phone; same with HUAWEI.

Result. I really need to achieve the compatibility of that soccer player's video with my videos and get them to work on, not only some but on all android devices.