It now appears to have resolved itself, somehow! Yesterday I tried the same steps with a different image and got the same result, and then again today I repeated the process and got the same result once more, but after trying it one last time with the original image and brush settings... Everything worked perfectly. I’ve now tried it with a handful of images and several different brushes since then and it has worked exactly as intended every time.
I have not installed any program or system updates in the time since it first started happening, nor have I changed any settings anywhere in the program, so I have no idea what was causing it or why it’s stopped... But all’s well that ends well, I guess! Thank you for your help!