Custom help pane currently only support Microsoft Stream (Classic), according to this MS doc: https://learn.microsoft.com/en-us/power-apps/maker/data-platform/create-custom-help-pages#video-and-static-images

However, Microsoft Stream (Classic) has already been fully deprecated and Microsoft Stream videos does not play in custom help pane anymore.

Please update the video playing function to use the modern Microsoft Stream infrastructure. This would greatly help in referencing corporate private contents.