For anyone stumbling on this thread later:

I tried all possible rom combinations (USA/Europe, Tosec/Redump, etc) and none was working for these 2 games. I started trying to change some Retroarch settings (ex: high level emulation, windows CE, etc) for Flycast and the games started to work. I thought it was the settings, but when I reverted everything to see which one exactly made the difference, the games still worked...

So basically, solution is: touch a retroarch setting, restart the game. Once the game work, revert the setting you changed, and the game will continue working afterwards, even after a full shutdown. 🤷