The length of the arcs (compared to the very short trailing of the actual star images over each 30 second exposure) suggests that these are trails of actual moving objects.
Welcome to the Starlink era. While your phone isn't (yet) set up to use the Starlink network for communication, it appears it's well capable of detecting the satellites by their reflected light.
I can't be certain these are Starlink birds, of course -- but they're very numerous, in low enough orbits to move significantly in 30 seconds, and even the newer, "darker" ones reflect enough light to make them comparable in brightness to dimmer stars.
This kind of thing is why some astronomers have been complaining (since the second or third Starlink launch) that this network and its competitor(s) would be the end of ground-based astronomy, even as we approach the ability to build telescopes (or telescope networks) capable of resolving surface features on exoplanets.
Commenters have suggested (sensibly) that there are too many to be Starlink, and rather than criss-cross straight lines, these are arcs that seem to center around a point, so the other significant likelihood is that one or more of the exposures that were stacked contained rotation of the camera (phone), resulting in images the software couldn't merge with the others. If you can go through the images one by one, it should be very easy to spot the ones with long arcuate trails from among those with near-points (you might even be able to see the arcs well enough in large thumbnails, rather than have to open each file). Eliminate those files, and you should get an image without the arcs.