My monstera put out a leaf with zero splits and I finally figured out why
I have had my monstera for 3 years and last month it pushed out a new leaf that came in totally solid, no fenestration at all. I keep seeing people say you need to wait for the plant to "mature" but mine is 4 feet tall with 9 leaves on it, so age is not the issue. The real problem was light: I had it sitting 8 feet back from a north window in my Cleveland apartment, and once I moved it to 2 feet from an east window, the next leaf had 5 splits. I think most people blame fertilizer or humidity when it is really just photons. What did you change that got your first split leaf to show up?
My monstera spent two years in a dark corner because I kept reading that indirect light meant basically any light, and I nearly bought a grow light out of pure guilt. Moved it right up against my east window and sure enough, splits showed up within a month.