Pics are small, so assuming the pipe is clear and the flap is seating properly, and check looks installed the correct direction, I would assume it has something to do with the additional flexible coupling increasing the size of the check valve's flexible coupling. Where the mating of the flexible coupling on top the other flexible coupling cannot be tight. The PVC should be 1 1/2" which the check valve coupling should be already sized for by itself.

I matched the old size with the new I currently have installed.

The pipe also looks a bit crooked from the side. I noticed that and put some boards behind it.

First question is from where on the check valve is the water spraying? Top, bottom, side???

It’s not plateauing anymore. It’s just looking from the top. Scroll up a little and you can see the pooling at the top at the rubber part.

The pipe isn't properly supported. It's pulled away from the wall and the stress is on the flexible connection. Sump pumps tend to cause unsupported pipes to "jump" and move when they kick on and off. Your "repair" with that wood is completely bogus. That connection will fail again and could be a real mess if it comes apart completely.

You need to determine why it's pulled away from the wall. My guess is that there is a horizontal run at the ceiling that's unsupported also, and bowing down - causing the pipe to pull away from the wall. A photo of that would be helpful.

Link to J hangers

Link to Two Hole U straps

Support the pipes correctly and then tighten the clamp at the top. Again, if you don't fix this properly you will have a mess.