How can I prevent Writer from auto-correcting "themself" to "themselves"

I looked in Tools → AutoCorrect options… and removed it from the replace list but it still does it. This is incredibly annoying as I write the word “themself” all the time.

I looked in Tools → AutoCorrect options… and removed it from the replace list

Tested under GNU/Linux using v5.1.2.2. Works exactly as indicated i.e., once the “themself”/“themselves” entry pair is removed from the list the term is no longer automatically replaced. No need to restart LO. Perhaps you have created a separate user entry in the user profile? Try resetting the user profile and see if the problem persists. Also, make sure that the language being typed in is the same as that for the AutoCorrect list from which the entry pair was removed.

In similar cases my advice was to not remove the predefined replacement, but to edit it to the effect of replacing the string under discussion by itself.
I tested some cases, but not this specific one. This is just a workaround, of course.

Editing
First follow the advice by @oweng. The examples I mentioned here did not concern “standard writing” but very specific replacements (fractions, more generally: typographical replacements organised with the help of the replacement table). In some cases at least, I think, the issue was not just caused by different language settings. This deduced from the fact that my workaround undoubtedly worked. I did not check if there was a bug meanwhile fixed.