So I was going to separate each and every error I found through my latest play through, but I have a feeling this is all tied together by something - and you'd get like five different bug tracker submissions at once. There are a couple of major errors that seemed to stick throughout, though, which I'll explain further down. I will label them all for simplicity, and I will leave the extra details, such as the version at the very bottom.

1: Broken Trait Pages
Describe the bug
Clicking the arrows in the editor does not change the page of the traits. This means I am permanently stuck on page one but I am able to select traits (thankfully).

To Reproduce
Steps to reproduce the behavior:

1. Enter the world for the first time.
2. Click on traits in the MCA editor.
3. Click arrow buttons.

2: Broken Command (/mca editor)
Describe the bug
I suspect this is a compatibility error, again I'm going to figure out with what at a later date. Buttons to change features do not appear, and all I am left with is a text box, a gui background and my supposed model of my character. I say supposed because this is going to link into bug 3.

To Reproduce
Steps to reproduce the behavior:

1. Enter the world for the first time.
2. Do the whole MCA editor set up thing.
3. Open chat box.
4. Do /mca editor.
5. See error.

3: Broken Startgame Editor
Describe the bug
As the name suggests, one specifically important feature that you get upon the creation of a new world breaks immediately. That feature is the ability to customise your skin model, which links back into the old one. The entire set of three buttons that you select your model from are gone, but you can still edit your character... except you're limited to your original player model and skin, which appears instead of the one you made at the start. To simplify as I see how weird I've worded this is; the game creates the model but doesn't use it, at least not correctly.

To Reproduce
Steps to reproduce the behavior:

1. Enter the world for the first time.
2. Complete editor.
3. Go to third-person.
4. See error.

4: Broken Bouquet Gifting
Describe the bug
Now I did see that someone also reported this further back, but also with proposing; for me, I've only been able to try with the bouquet of flowers I made 13 of. I interacted with 4 NPCs, one of whom who had over a 100 hearts and still could not get a relationship as an outcome. Instead, I got rejected every time without fail. I could even perform the kiss interactions and stuff, but I still got rejected. I can't remember what traits I selected, but considering I can't change them because of the editor issue I'm facing, it looks like I might be stuck on this world. At least until a fix.

To Reproduce
Steps to reproduce the behavior:

1. Open a new world.
2. Do the whole mca editor thing.
3. Build a relationship with a villager.
4. Make a bouquet.
5. Gift the bouquet.
6. See error.

Version
7.3.21, 1.16.5, Forge 36.2.39