Perhaps I don't have full understanding but I think there are a couple of glitches.

I was on with Herman and went to stack a couple of Ish protects.

as I pulled from Depot, I noticed one stating OLD

when I tried combining them the first two appeared to stack, then when I tried a third (I thing the OLD one) The pro went to an appearance of the Silver armor to the MS Bronze armor.

Also the "5" (?) went to a red blotch.

Yeah something similar with Protect.

I had 2 Scrolls Stacked
Went to stack a 3rd scroll and it 'used' my 2 Stacks turning it into 1 Stack with 2 charges.
Used it again and its now 1 Stack with 1 charge (then gone).
Issue being its completely lost a stack.

During this my inventory was full, so if if it was supost to break it apart that might be the cause?
Tested with empty inv, def resets stack to 1 when using OLD scroll on stacked scroll

Graphics file is also broken.
When its a 1 charge Protection scroll its showing me the MS Scroll picture


Replicated with an Ankh Prot Scroll..

I'll just add my experience as well. Playing on a character named 'Home.'

I started with a nearly full set of 5x3 ish spell stacks (bless, prot, ms, curse, and enh at 3x3, I believe), and tried to spell up. This was my first time using the stacks, and I didn't realize they would try to unstack. Unfortunately, I had a full inventory. I quickly clicked the 5x3 bless and 5x3 protect, before realizing that something was wrong. I ended up having a bless get casted, but there were no longer any bless scrolls in my inv. No protect was casted.

Figuring that the problem was the full inventory, I went to the depot and cleared some space. I also grabbed another stack of 5x3 bless scrolls and tried to continue spelling up. Protect next - I found a 1x1 protect in my inv, which I found a little odd as I didn't recall having it before, but figuring I was just mistaken and that I should go ahead and use up a nearly-finished scroll, I went ahead and used it. It cast bless on me rather than protect, and disappeared. Ok, I guess that scroll had gotten generated as a glitch previously.

So now thinking that I had cleared all the traces of the earlier full-inventory issue, I went to the protect stack which split and casted perfectly. Then thinking all was well, I quickly clicked the 3x3 enh and also clicked the 5x3 ms to unstack them. The resulting effect was that rather than unstacking, I received a cast of enh and ms. Also, I now have only a 1x3 enh and zero ms scrolls in my inventory.

Upon carefully checking what I now have, I can see that I have a new 1x1 scroll with a curse sprite and ms description, and a 1x1 scroll with prot sprite and enh description. I guess these were produced during unstacking the enh and ms. I also would guess that these correspond with the strange 1x1 protect I had used earlier which in fact casted bless. I'll hold on to these glitched stacks as a record for now.

I'll hold off on using any more stacked ish scrolls for now. And I guess I should count the 5x3 bless, 5x3 ms, and 2x3 enh stacks as lost?

I was not able to recreate the vanishing stack trick as that may be beyond my magical capability.

But the incorrect sprites for some used scrolls I found and was able to recreate.

This should now (hopefully) be fixed.

If you still have problems and are able to recreate the problem then please let me know.

Thanks for looking into the spell stacking! Everything has been working smoothly on my end for several days now.