How do interim or partial transcripts work for application, and is it possible to have an admissions decision reversed based on the final version? I attempted to be as detailed as I could in the title, but let me elaborate a bit more.

I'm sure this situation is common (if not normal) but I'm applying to PhD programs this year fall and will only have been done with half of my master's program by then.

In cases like this, would I submit the transcript for the first two semesters of my master's degree? If I were to receive an admission green light and confirm to enroll (which is usually around April I believe), is it common to have these decisions reversed if my final GPA were to fall?

Thanks for any feedback or tips.
You need to explore this with the institutions you apply to. 

And you need to read the correspondence carefully. If you get accepted, then you are probably accepted except in rare circumstances. But if you are "conditionally accepted" then you need to meet the conditions, whatever they are, to assure acceptance. 

Acceptances are generated based on more that GPA, of course. Someone needs to make a prediction of success before one will be issued. But no prediction is perfect and students fail out even after enrolling. Universities know these things. 

Tips and advice: Work hard. Don't let the bad things happen.