I take it you are trying to get the going thing to be in front of the blue letter. I would use two copies of your green thing layer and put one on top. This should also make masking much easier.
It's really hard to tell what you are trying to do. If you want to see exactly what the mask expansion is doing then solo that layer and enable transparency grid or put a solid below it. Without some more explanation it's really hard to see if anything unusual is happening.
Maybe this project file will help: Dropbox - Threaded Vine.aep (Note: your browser may add a .txt extension to the file - just remove it)
In this project I used a shape layer with a very wide stroke to create the background for the vine. I then drew a mask to taper the stroke. I was thinking about adding the Trim Paths shape animator to make the vine grow but I used Mask expansion instead. Then it was just a matter of pre-composing the growing vine and putting the text between the layers. I then used very simple rough masks to make make the Vine FG layer appear in front of the parts of the text where I though they would go. EZ PZ...
The entire animation was done with 2 keyframes in about 5 minutes. It actually took longer to convert the movie file to an animated gif in Photoshop than it did to do the animation. A few more minutes in AE to or better yet, using Illustrator to create the taped vine and mask and the animation would be a lot smoother.
I tend to agree with Rick. I watched your video an hour ago and chose to not reply because I don't really understand what I'm looking at. At least for the positioning issues you might want to consider pre-composing. Sometimes effects mess around with the transparency buffers and layer bounding box/ paint canvas and as the effect changes, so do these things and in turn other operations applied after that will give (seemingly) odd results. Pre-comping would avoid that.
Sorry about not being specific. I had thought it was just about the mask expansion property seeming to "break" (no longer effecting) when I set the keyframes. Also the masks were not accurately respecting the mask paths...as if I had made changes to the mask expansion when i actually hadn't although I did not include that in the video.
Rick i will take a look at that project file later but here's the specifics of my project:
The text is AE native text & gets revealed with write-on effect so it looks like it's being hand written.
After it's fully revealed the vines come up, which are Trapcode 3d Strokes applied to masks. (so maybe the masks being 3d layers and the text NOT being 3d is causing trouble?)
As Rick guessed I was trying to get the vines to pass over some parts of the text and under others. So I applied masks to the text to hide it where I wanted the vines to pass over. This was not the best solution as I was forced to try and animate the masks since I needed them to have no effect until the vines passed over them. (I probably should have pre-composed the vines & applied the masks to the pre-comped vines where I wanted them to pass under).
Possible solutions aside...what was truly bothering me was what was causing the mask expansion property to behave differently than I expected it to. As you can see @ 00:53 of the video I uploaded, the mask expansion property works until I set an initial key-frame. After that, when I adjust the mask expansion value it has no effect.
Pre-composing the vine & masking the comp was sooo much smarter (less stupid) than my dumb head trying to animate masks on the text. It took me about 45 seconds to get perfect results.