Thanks for reaching out. If you can clarify a few things for me, we can get to the bottom of this.
Could you please share the version of Lightroom mobile & the Android OS on your mobile? 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?
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.