The filters, sort order, etc are remembered for each folder/collection/etc.
It sounds like you're expecting the Sort order to be "File name" (it's easy to set it: you do have Sorting enabled in the Toolbar at the bottom of the grid don't you?). By default it's probably "Capture Time".
Are these new TIFFs getting the EXIF data stripped from them? If so, the equivalent field will be getting written by Photoshop and will end up effectively being the date/time you first saved the TIFF. Thus sorting by Capture Time might result in the effect you're describing.
Or is the sort order set to Import Order? It's the top image in each stack (by default the new TIFF) that is used in sorting, and thus they go to the end.
Thank you for your reply.
The sorting is by capture time.
I checked the capture time of the TIFFs. They are the same as the original RAW file. PS has not overwritten the capture time.
PS: the weird thing is, that not only the new TIFF shows up at the end of the folder. It drags the DNG down with itself, rips it out of the sequence, and now the DNG is also at the end of the folder.
Currently I work around this by giving any chosen image its own subfolder.
But my confidence into the library module is shattered, and now I back up daily. By hand. Copying image folders from the main hard drive to the external.