It's happening on both v2 and v3 (I first discovered the game on v2, then found out there was an update, so I tested both).
Edit: For some reason, it randomly saved an Onslaught high score.
Viewing post in Added Pico8 files and fixed a crash comments
I broke my high score, and it saved it again. I tried to find a logic to it, but I couldn't. I saved a state right at the end screen, and if it doesn't want to save the score, it won't.
By the way, not sure if related, but on Onion OS (fake-08, by the way), selecting "Try again" does the same as selecting "Quit"—it just sends you to the start screen.
Oh, if you're using fake-08 then that is probably why there are issues with the saving data like highscores (it's also probably why "try again" just reboots the game instead of restarting the game mode you just finished). I'm not sure if I can fix it in my game as that may just be an issue with the fake08 emulator. If there is a way for you to run Pico8 natively on your Miyoo Mini Plus that would probably solve the issue, but if I find out another work around I'll let you know