I don't have any learning disability that would cause difficulties with spelling/writing. I am close to what is sometimes called "absolutely literate" or "innately literate" in Russian literature on autism. If I am attentive to what I am writing in the three languages I am fluent in (Russian, English, Lithuanian), I make no grammar/spelling mistakes whatsoever. I am also picky about using the right grammar in languages I've started to learn recently and am reluctant to use words or phrases when I am not sure whether they are correct or not.
However, it is all too easy for me to stop paying attention to what I am writing. It happens at the slightest distraction, or whenever my thoughts start wandering away a bit. Then I start making the oddest spelling mistakes - I "spoonerize" my spelling, transfer letters/sounds from one word to the next (as in those classic examples, "black bloxes" for "black boxes" or "Noman numeral" for "Roman numeral", etc.), join two words into a single neologism, want to write one word and end up writing another that sounds similar or begins the same way (for example, I might want to write "were" and write "where" instead, or "anything" instead of "anyhow", "paint" for "plant" etc.). I have no idea if this is ASD-related or not, but it probably does bear some connection to how my brain is wired. I remember we had a psycholinguistics course as part of our English philology program, and at some point the lecturer told us about various "slips of the tongue and brain". I just sat there and thought, oh dear, all this is about me.
When I have no time to mind what I am writing, for example, when I'm writing too fast - say, if I am at a lecture and am trying to take notes of everything that is being said - what I put down is practically unreadable. Illegible handwriting aside, it probably looks like something written by an aphasic.