When I type into Libre Office Writer the Auto Correct does not work. So, if I forget to begin a sentence with a capital letter, Writer does not do that for me. If I type the word ‘i’, Writer leaves it like that, rather than changing it to ‘I’.
If I look at ‘Tools’ I see that Auto Correct is checked - and if I look at the Auto Correct Options, everything is as I would expect.
I imagine that sometime recently I may have fat-fingered Ctrl-something instead of Shift-something - but there are rather a lot of possibilities to try to see if I can back out that way.