I recently came across this problem in our email tech support, and it turns out this (and potentially other CSS-related issues) are a known issue with the .mobi files KindleGen creates, and Amazon’s iOS mobile apps.
There is thankfully a simple solution, and that is to load the .mobi into Kindle Previewer 3. You will note that it takes a little while to load the file, claiming to be “Converting your book to kindle format”, which is a bit weird since Mobi is a Kindle format. But what it is doing is generating an AZK file on the fly, which it will use internally when previewing tablet and phone devices in the software. That’s the file you need to put on your devices, not the Mobi.
To get it out of Kindle Previewer, hit ⌘X or use File ▸ Export…, and save the file using the “Books - for side-loading to iOS devices (*.azk)” file format.
As for KDP, continue using the Mobi file as compiled out of Scrivener. The problem mainly has to do with that .mobi file being tuned for KDP upload, rather than being read like a normal book on devices directly.