New Rear Wheel goes out of true
It's normal for a new wheel that was under-tensioned, unevenly tensioned, inadequately stress-relieved or whose spokes are twisted.

Either that or it's being abused - ie. if you're really throwing it around, or are carrying heavy loads on the 24-hole version.

The repeated re-truing may end up with a decent wheel, since you're just performing the stress-relieving step while riding instead of in the workshop. It may equally damage the wheel before you get there, and you really shouldn't have to pay to fix a bad build.