Endless problems with Photomerge

I have read much on my problem but can't find an answer that helps.

I am using Photoshop v21.2.1, on a PC which is relatively fast and having 16GB of RAM.

My needs are to scan a largish document, similar to a family tree, ie, lots of lines and text which should leand itself to photomerge  To scan I use a flatbed A4 scanner and to ensure 25% overlap, I need 6 scans, 3 across and 2 down.

If I take all 6 jpg images, and use File, Automate, Photomerge, and äuto", I get very strange results which resembles planr rectangles joined in a 3D manner, resembling a house that someone tries to male with a pack of cards.

If I then take just 3 adjacent images (the top row of the 3 x 2 matrix) the Photomerge function works well.  Doing the same with the next row, the merge again works well.  Then if I try to merge these 2 resultant .psd files, I get a crazy 3D-like merge.  Similarly if I photomerge the first row of two images, that works all OK.  but when I then try to photomerge these 3 column merges, I again get crazy results.

A few years ago this very same exercise worked OK, and if I did have issues I could correct them by using the "reposition"option, but when I do this with V 21.2.1the reposition continually plonks adjacent images on top of each other which are shown as two layers (partial images) as it should but I can only drag around as partial images, making the whole exercise useless.  Maybe I'm not clear.  When I select a layer to move (with the reposition setting I can't capture the whole image to move.  I can only drag fragments.

Some other details that help are that I am scanning at 300dpi, and saving to a jpg.  The individual jpgs are about 2.5 Meg.  When the PC is working on Photomerge, the RAM usage stays below 40%, and CPU is at a lazy 30%.  

I am not a power user, and probably don't deserve a package like Photoshop.

I hope someone can help.  Stay safe people, in these troubled times.

Doug