This isn’t a bug. There was a very contentious debate on the old Google Group about capitalization of common names. We decided to, on the website, display common names in a certain style regardless of how the name was entered into iNat’s database, but chose to not extend that style to the mobile apps. Whether or not that last part was a good decision could be debated, I guess, but it’s not something we’ll prioritize.
Many plant common names are entered in all lowercase on iNat because that seems to the standard in North America at least. But please don’t discuss that on this topic.