The legend functionality in Power BI core visuals is far from ideal, but its probably slightly worse using Vega-Lite in Deneb.
The solution is one you might not expect, but it solves the issues we get with both core visuals and Deneb.
𝗟𝗶𝗻𝗸 𝗚𝗶𝘁𝗵𝘂𝗯 𝗳𝗼𝗿 𝘁𝗵𝗲 𝗩𝗲𝗴𝗮-𝗟𝗶𝘁𝗲 𝗰𝗼𝗱𝗲:
github.com/powerbiferrytales/Deneb/blob/main/stacked%20bar%20with%20interactive%20legend.json