Loose spokes after truing a wheel

Jobst Brandt's The Bicycle Wheel tells about repairing this failure mode in the following way:

In a soft failure the rim has a smooth and continuous bend, and some of the spokes have become loose. If the wheel is deformed laterally more than ten millimeters, all nipples, even on spokes that have no tension, should be unscrewed two turns before attempting to straighten the wheel. Straightening the rim by bending it without first unscrewing the nipples would, in effect, straighten and tension the wheel at the same time. Straightening makes the rim yield and take a new shape. When yielding, it will deform not only in response to the straightening forces, but also to the spoke tensioning forces. If the nipples are not unscrewed, the combined forces would cause kinks in the rim at each spoke.

To repair such a bent or collapsed wheel, first loosen all nipples about two turns (one turn if the wheel was loosely spoked). Lay the wheel on its side with the largest wave toward the floor and press down on the rim on either side of the bend. If necessary, repeat this process turning the wheel over until the rim is reasonably straight. Now place the wheel in the truing stand for tensioning and truing. If the misalignment is still more than ten millimeters, repeat the procedure on the floor while gradually tensioning the spokes. Now the wheel can be trued as previously described. Because straightened rims usually have nonuniform spoke tension, the wheel may not stay true.

I suppose you didn't follow that procedure but rather trued the wheel in a truing stand only? Loosen the nipples as directed, straighten the rim and retension and re-true.

A rim which is not straight when at rest, will result in uneven spoke tension after it's trued. This is always the case, because no rim is perfectly straight.

If the rim is only slightly bent (the 10mm figure sounds reasonable), then the resulting un-evenness in the spoke tension will be moderate and tolerable. But if the rim is excessively bent, or if it's very stout (such as some BMX rims), then it will not be possible to true it without excessively uneven spoke tension. The rim should be straightened or discarded.