"هٰذِهِ قِطّة اَلْمُهَنْدِس."
Translation:This is the engineer's cat.
This cat is an engineer: هذه القطة مهندسة
Notice where the definite article (AL) is placed. If I switch (AL) to مهندس, then مهندس would not become a predicative anymore but rather signifies a genitive relation to the other noun attached to it (قطة).
And by the way, in my translation above, I've changed مهندس to مهندسة to fit the gender of قطة here. Since we moved (AL), then this word becomes predicative, and the predicative typically describes the subject - which implies it must coincide with the subject in gender and number.
It sounds alright for me here.
This said, if you are expecting Al-Muhandis (with A pronounced clearly) then this is not the case here. The "A" in the definite article AL is what we call Hamzat Wassl همزة وصل (ironically this Alif has no hamza like this ء). It means (Connecting hamza). It works somehow like a schwa and sometimes vanishes completely actually like in this case here, where the last vowel of قطةُ (Qitt^atu) gets connected to the next word (Genitive relation here) to form: Qitt^atul-Muhandis. The Alif disappears here though written.