Hi Paul,
Thanks for your feedback.
I've deleted the 2 files in question and there's no longer any problem with the "Button" addon, which displays the normal and hover styles :)
Concerning the Flex template, I started using it about ten years ago and I find it quite extraordinary in terms of the possibilities it offers, but it's true that it can cause certain problems.
It's thanks to it that I discovered SP Page Builder, which I use every day now.
Since then, I've been gradually replacing it with the Helix3 template, even though I can't always get the same rendering as with Flex, especially when it comes to headers ;)
Have a nice day!