I concur with two possibilities above. That there could be cracks around one or more of the nipple holes which the spoke/nip is slowly pulling through and preventing the proper tensioning. Nothing really can be done about that. Rim needs replaced.

Or, the spokes could be twisting, and then settling once the wheel is under pressure while riding. It this is the case, once you true...ride the bike down the block and back to let them settle. Sometimes you can hear them settling in the first several rotations as you roll away. The spokes with pop and ping. Then, go right back home and re-true. Sometime it may take two or three of those evolutions.

You can also prevent the spoke twisting by holding it in place with smooth pliers as you twist the nip. https://www.wheelfanatyk.com/blog/wh...spoke-wind-up/

Dan