adding drop shadow to a text, creates a bug with the Typewriter effect. Instead of words coming out letter by letter, when drop shadow is activated the words come out in chunks or in full sentences

The Typewriter effect isn't native to Premiere; it was added following the Film Impact acquisition. It works based on alpha channels, so adding a drop shadow causes the text to be treated as a single unit rather than individual letters. This is how it works, not a bug.

There is a workaround, nest the text layer after applying the Typewriter effect, then add the drop shadow effect to the nest. This should give you the result you're looking for. I hope in the future Adobe improves this effect and works no matter what you toggle.