The correct pronunciation would be:
almuhandisatu muhandisatun jadiidah
If there was another word following the last it would be:
almuhandisatu muhandisatun jadiidatun (huna)
The taa2 marbuutah is pronounced in the first and second word in either case, and in the second with nunation.
So, I posted this as a separate question in the discussion forum and got an enlightening answer. Apparently, ل and م together form a common ligature (=combined form of letters) – ﳌ – that appears in some fonts and not in others. ﻻ (lam + alef) is obligatory in all fonts, but I'm not sure which other ligatures are used in common fonts. Guess, it's just a matter of finding out when reading…