I have a pre-existing world that is pre-13, then used and optimized for 19W11b.  When I leave and return to a chunk that has 3 clocks placed on the wall, I find the clocks and frames on the floor as separate items, no clock or frame on the wall. The wall is unchanged where the clocks were placed.  If I then save the world, close the launcher, and restart the world, the clocks are back on the wall, but the fallen clock and frame are still on the floor as items.  If I just close and reopen the world, without closing the launcher, the fallen clocks and frames are still on the floor, but they do not reappear on the wall, only if I close the launcher as well.