I have had the s10 for over a year and tried just about everything to get the keyboard to autocorrect well. It is atrocious, it keeps getting mixed up between the French and English keyboards, and autocorrecting words that are correct with the strangest constructions.
My aunt recently bought the s10e and is having the exact same issue.
Also the Internet is full of articles with the same issue with the s10.
This isn't a Galaxy S10 problem, its a Samsung keyboard problem. Personally, i dont like the samsung keyboard, so i use a 3rd party one called Microsoft SwiftKey keyboard. You can download it in the app store and has much more customisation than Samsungs keyboard. I would recommend that, but you can switch to any other 3rd party keyboard and each will have their own prediction system and dictionary.
Thanks Shazzkid. I had the s9, so and s5 and the keyboards worked perfectly well, so that's why I am inclined to think it is a problem with s10.
Thanks for the advice about SwiftKey - my only reservation is that SwiftKey has very poor confidentiality protection, i.e. anything that you type (including passwords) can be easily be read by third parties...