sourceRectAtTime expression has unpredictable results when used in a mogrt.
This happens a lot. I use the mogrt and everything seems fine and I render my video from Premiere. Then the layer that has the sourceRectAtTime expression will start jumping around the screen and appearing/disappearing. I restart Premiere, and sometimes it's back to normal, sometimes the jumping around is different. Every render is a surprise.
I've been building morgrts since the start, using these expressions, but since AE 16.1 update, it's been buggy with unpredictable results in Premiere.
Also, if I reopen a comp in Premiere that has a mogrt that uses the responsive time feature, the clip is sometimes missing the outro animation. And I have to set up the mogrt all over again. Before saving and closing there would be an outro to the animation, but it just disappears sometimes if I reopen a project.
That sounds super frustrating.
I haven't seen issues like the ones you are describing with sourceRectAtTime. It would be super helpful to get your mogrts and a project that is exhibiting this behavior. I'll reach out directly via email shortly.
We currently have an open bug on issues with responsive time that sound like they might be related to what you are seeing but I'd like to know more about your set-up to see if what you're seeing is the same flavor of bug or something different. Something we have learned is that these mogrts might get retimed unexpectedly if they have been used on a different project and are being relinked to the timing of that version.
Shooting you an email now and hope to get you back on track.