Externally edited photos won't stack with original in LR Classic

I am using LR classic v9.4.  My workflow includes using tools in the develop module to get an image (imported as a DNG) to close to a final edit, after which I use the edit dropdown to finish the edit (mainly sharpening and tagging) in PS.  From PS v21.2.3, I will save the file and a high-res JPEG file on an external hard drive.  These steps typically result in the final saved image being stacked with the original in LR.  However, in the middle of an editing session in which I followed the above steps as usual, I suddenly noticed that my saved edited photos were no longer being stacked on the LR filmstrip alongside the original as had happened earlier in the day.  I checked "external editors" tab under preferences and found that "stack with original" was checked.  I unchecked and re-checked it, but the saved images still didn't stack.  Restarting LR also failed to correct the issue.  Any suggestions?

Thanks, Aaron