Hello there!
Let me answer to both posts, which are 2 different situations.
1) Wall feature can not be displayed as Mosaïc natively, you'll need a third-party app or to use Videolan Manager in order to do that in the way you want it displayed.
Here's some steps to help you with Videolan Manager :
https://docs.videolan.me/vlc-user/deskt ... vlm-mosaic
2) For windows disappearing when launching video with Wall Feature, can you please try to disable Graphic Acceleration on the Settings?
The way to do it :
1. Open VLC and go to 'Preferences'.
2. Select 'Inputs / Codecs'.
3. Click 'Hardware-accelerated decoding' and select 'Disable'.
4. Launch the media with Wall Feature enabled.
Let us know if it works! Have a good Day!
Regards.