Sorry about the import issue experienced with Lightroom Mobile. We're here to help.
Where are the CR3 RAW files stored, on the Android device storage or an external (OTG) storage? Could you please share the CR3 samples with us? Is it happening with just CR3 file formats? Since when are you seeing this issue?
When you import images on Android, the Android File Manager can open and might not load the RAW images properly to show you previews. The default Android File browsers are not fully equipped to handle RAW photos. Please import pictures in Lightroom and then check if they are loading correctly.