Hovering over the buttons also didn't change their state.  I think it's related to dismissing the tutorials because the introduction tutorial popup covers some of the unclickable buttons (save and load). Sometimes I would click and a popup would ask if I'd like to disable tutorial tips. Eventually I did and all the buttons worked again. No errors in the console.

Edit: I confirmed by adding prints in the source that the tutorial buttons (except for the dismissed one(s)) were still capturing input after being dismissed.

Output:

show_tutorial_window - introduction
confirm_message
hide_text_window
show_tutorial_window - slavetab

How to reproduce:

1. Restart Strive and create a new game and characters.

2. Click the introduction tutorial button and dismiss it.

3.  Click one of the characters in the population box then close the character screen.

4. Click where the tutorial buttons should be located.

5. Open the menu and you will be unable to click the save and load buttons if you clicked a tutorial button in step 4.

Quick fix (disables tutorials):

Open /src/TutorialNode.gd and add "return" after line 24 so it looks like this:

func rebuild():
    return
    if state.show_tutorial == false:

Or just hide the tutorials instead of dismissing them.