"robustest" Why do all grammar software underline this word? [closed]

I wrote a sentence like the following using the word "robustest". But many grammar software started showing typos. The word exists in the dictionary. Why is this happening? I came across it in an article I wrote using American English. Thank you for your valuable feedback.

Finally, classifier models are produced. Performance tests are performed on models and the robustest model is tried to be synthesized.

Solution 1:

They want you to use "most robust".

Most multi-syllable superlatives don't take "-est", but rather use "most ____". https://www.grammarphobia.com/blog/2018/07/comparative-superlative.html