This has been an issue recently. The generated slides no longer replace the accent images once the theme is switched to another. It has been annoying to replace, and even the AI Agent would miss some slides if you use that.
Check attached screenshot: theme was replaced from "Linen" to "Shadow" yet the slides on the Shadow are still using the colors from Linen. After using the AI Agent, it only replaced some of the slides and had to go over it again to replace everything entirely.