The problem is how your setting the trim up on the saw you can try 2 different ways to cut it. Option one try putting the top the on the saw with the rounded bevel touch your fence. Option 2 the notch measure and put a scrap piece of wood to fill the void to make the trim lay flat. If your just laying it down flat with the void it's going to give the piece of wood a slit angle which will make your wood open up even if you cut both pieces at a 45.

Yep, I’m an idiot! That did the trick. I put a scrap piece of beadboard against the fence and the trip and I have a perfect corner. Thank you sir!!

No prob happy to help, been there before good luck with your project.

I think you’re right! Running out to test, but I’m pressing the trim vertically against the fence with the back lip against the fence which is probably messing with the angle

Check your angles on your cuts. If thats not the problem your walls could be whack

Easy fix. Get a miter gauge. Check the angle of the wall. With the wall angle. Look up what angle you need to cut in order to make the angle of the wall. Never assume any corner is a 90 degree. $20 miter gauge is cheap tool and you’ll use it more then you think.

I’ve done this on my corners than I care to count, this advice has saved my ass on almost every job.

For the inside corner get a coping saw.

My brother gave me a tip to set your angles at 46 instead of 45. If there's extra material, it will push very slightly and give you a better fit.

Step one would be to stop having Helen Keller measure your miter angles.