For Dynamic Background when options
"Hide background for no maximized windows" is enabled
and "Monochrome contents when panel is transparent" is also enabled
enforces transparency in somes cases but there are backgrounds that are too busy in order to provide enough contrast. As elementary approached this : https://medium.com/elementaryos/elementary-os-5-juno-is-here-471dfdedc7b3
the background is considered Busy or not. When the background is busy then is such cases a transparent background is shown