Another test would be to delete the thumbnail preview and recreate it... this seems to have come up a few months ago: https://community.adobe.com/t5/lightroom-classic-discussions/library-standard-preview-not-updating-for-iphone-15-heic-files/td-p/14222470
Please take a look at the recommendatin from @Jao vdL
"Make sure you set the default for development of images to not do HDR. In Classic, this preference is in Preferences->Presets. On the phone version of Lightroom that is in App Settings->Import. "
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