How are you making sure you use the same zoom level? There's no indicator for zoom level, and it's hard to not accidentally change the zoom while zipping around the screen multiple times.
I've done this a couple times and 1 problem I had to deal with was the upper right info box that shows the armies/money amounts ended up becoming part of the map. Same with the level name. I was able to get around that by increasing the overlap between the maps being stitched together, notably ensuring that the image being stitched on the bottom has enough vertical overlap to cover the level name and army/money box entirely.
I did all mine manually but there should be software that does this. Doing a quick search, Hugin seems to a free, open source option:
https://hugin.sourceforge.io/And there's other paid options like Photoshop, etc. Not sure how they would handle borders, above mentioned army/money boxes, etc. It's likely best to just screenshot within the borders, excluding everything that isn't part of what you want in the final image, being careful to not include level name, army/money box, icons at bottom of the screen, etc, while also having enough pixels overlapping so that the stitching process is both easy & accurate. And as Hugge said, clicking somewhere off map so you don't end up with a stitched together with dozens of territories highlighted. Basically click somewhere that causes the icons at the bottom of the screen to retract (rather than popup and provide info). If you click a territory, then click into the middle of the ocean (no territories, no bonus markers), this will happen.
Edited 3/12/2023 15:25:02