How to re-enable missing Disclosure arrows?
Solution 1:
Weird little things like this can almost always be resolved by relaunching Finder. There's a number of different ways to do this, but I usually ctrl⌥-click the Finder icon and select "Relaunch".
When that doesn't work, the next step is to turn the computer off and on again. Basic, I know, but there's a reason they're always mentioned.
Some musings from a former Apple employee on why restarting is effective: https://techreflect.net/2018/04/20/why-does-rebooting-fix-everything/