All my schedules work just fine except in this one old project that I exported some newer schedules to. So it's no big deal making a workaround for just this one project:
1 Give all objects in that schedule an Element ID prefix,
2 Set that up in Scheme Settings, and
3 Take out the scheme setting about layers.
In schedule View Settings, btw, Layer Combination is greyed out. If I start in plan with the right layers on, then Go: Last Schedule and open View Settings, Layer Combination has reverted to All Off (which happens to be the first in my list of Layer Combinations), and it's still greyed out.
AC fan since v 7. Currently on AC 26 Build 5003 USA FULL Apple Silicon,. 2022 Mac Studio, 32G ram. OS X 12.6.2