Angled cut saw blade deflection?

The most likely cause of inconsistent results is operator error.
To get repeatable angled cuts like that you have to make sure that you have each piece of wood identical. Check the worst angled piece and make sure that each side is EXACTLY 90 degrees to the other three. One degree out of 90 will angle the cut

Also, if you are new to the hobby, dont push the wood through too fast. This can make the wood, and even the blade, flex a little. This is a very big problem on bandsaws.
If you are learning from utube understand that many of those videos speed up the cut rate because its boring to watch. If you push too slowly the wood will show scorch marks, too quickly and you might end up with what youre looking at.

Something is out of square. This is called a TAPER cut. If your jig isn't perpendicular to the table you'll get the results you're seeing.

User error is highly likely. It was an awkward angle to hold snug via a push stick. Might screw a hold to the sled next time.

Looks like the stock is moving during the cut plus a bit of blade defection

I've seen this effect when making neck joints for guitars. I correct it with a hand plane or a sanding board.

You can check whether it's blade deflection or stock that's moved on your sled by making a second pass through the blade. If it was deflection, the blade will cut on the second pass. If there's space between the stock and the blade, the stock has moved.

It's also possible your blade is misaligned to not be parallel to the miter slot. That would tend to push stock so it moves. It's not good.

I think we could give you more precise answers if we could see a photo of the jig and how it works.

I agree it would help-it was just a piece of scrap plywood with stop blocks screwed in… that turned out to be a piece of plywood that was not scrap that my gf was using and I had to disassemble (and apologize).

Will post a replica though.

I would check for square edge on stock and parallel fence to blade on the saw. If you are cutting tall pieces on an edge, it's best to have a higher fence and yes a stop block at the rear. A simple fence to build is an "h" profile that fits over and straddles the saw fence.