Curly quotes are available in various styles, depending on the font used. Some fonts feature a straighter version that only slightly curves and tapers inward, making it difficult to distinguish from straight quotes.
Consider changing the font in your preferences on the Writing Tab to something like DP Sans Mono (designed to aid in proofreading) or EB Garamond, which will clearly display the difference. You can set your Writing Preferences in the Writing Tab by clicking the T icon at the top of the Right Sidebar. Any changes made here will only impact the Writing Tab and will not be applied to the final book formatting.