Custom folder icon won't appear on dock

I have various folders of Excel spreadsheets on my Dock with custom icons. But there's one, which I labelled "2019," that, when I drag it to the Dock, changes to an icon of a spreadsheet. When I remove the spreadsheets inside it, the custom icon, 2019, appears on the dock. When I drag the spreadsheets back into the folder, the icon on the dock changes to the spreadsheet icon, while in the Finder, the custom icon stays. I created a new folder, pasted the custom icon to it, which appeared, but when I dragged it to the dock, the same thing happened: the custom icon turned into a spreadsheet icon. As I said, I have other folders with excel ss's that retain their custom icons on the dock with no problem. The 12 spreadsheets in the folder are all more or less identical, each named a different month. Any idea what's going on?