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.