Something that I noticed while checking out 2.23.0, and with older versions of Plotly.js, is that saving the image of a heatmap using the built-in download plot as a PNG file does not include the heatmap image in Safari. Checking back with previous versions, including 2.20.0, the behavior exists there as well. It's included in the PNG download in Firefox and Chrome.

Since it seems to be an issue with WebKit, it would be a decent assumption that the issue would also appear on other iOS and iPadOS browsers that use WebKit as a rendering engine.

Screenshot: https://imgur.com/a/c4iCy9n
Live example (v2.20.0): https://graphs.wwdt.me/shows/monthly-aggregate-score-heatmap