This is usually not necessary to use a separate filter because the text filters have keyframable size and position as well.
This is the project that I’m trying to import. In the project, I see that everything is transparent. lower-third-template.mlt (18.1 KB)
It seems you uploaded the wrong file. This file does not look at all like a simple text project.
But when I import it into a different project, the sliding text brings in black background with it.
This workflow depends on Shotcut’s ability to output with transparency, which is limited: Any place where video is actively blended across multiple video tracks will be opaque.
Here is a simple example that works: lower-third-template.mlt (4.3 KB)