Word not being added to spell check dictionary

I am new to this software. When spell checking, it is showing a word as misspelled, I select “add” to add it to the dictionary, but next time it encounters the same word it again shows it as misspelled. In other words, selecting add does not actually add the word to the dictionary, and it keeps coming up as misspelled.

I checked the en-US.custdic file and the word is there, but it still fails to recognize it as not misspelled.

I see an old post of the same issue, with no resolution (Words not added to custom dictionary)

What am I missing?

Thanks