Two bloggers I follow loyally - although neither may realise it from the rarity of my comments on their postings - by coincidence both use only lower case for their identities.
marcellous has a legal and musical bent whilst wcs posts glorious photos from his adopted French home. Apple's spell check doesn't like them; at least in my experience. Whenever I type their IDs spell check stubbornly tries to change them to marvellous and was respectively. Even the links above were only accepted after fierce resistance by me.
I can see why Apple might try to change the entirely lower case marcellous. A capital M could be more to Apple's taste. But in regards to wcs I assume case is not the issue, rather that Apple doesn't cope with lesser known acronyms.