Nesting tcolorbox changes border style

I have a box with a modified border style, but when I nest two of such boxes, the style of the inner one is not the one that I set. I know, from the doc and from other questions on this stackexchange, that nesting breakable boxes can create problem, but I don’t remember having read anything about styles.

So my question: how can I nest boxes while preserving the style like if the inner one is not nested ?

documentclass{article}
usepackage[skins, breakable]{tcolorbox}
tcbset{
    enhanced,
    frame hidden,
    sharp corners,
    }

newtcolorbox{sidebox}{
    borderline={1pt}{0pt}{gray!80!cyan},
    colback=gray!80!cyan!5,
}

begin{document}
begin{sidebox}
    [A=B]
end{sidebox}

begin{sidebox}
begin{sidebox}
    [A=B]
end{sidebox}
end{sidebox}
end{document}

---

Notice that, in this MWE, I nest the same kind of box (i.e. a sidebox in a sidebox), but in practice, it is not necessarily the case, and the box might have been created with tcolorboxenvironment.

documentclass{article}
usepackage[skins, breakable]{tcolorbox}
tcbsetforeverylayer{
    enhanced,
    frame hidden,
    sharp corners,
    }

newtcolorbox{sidebox}{
    borderline={1pt}{0pt}{gray!80!cyan},
    colback=gray!80!cyan!5,
}

begin{document}
begin{sidebox}
    [A=B]
end{sidebox}

begin{sidebox}
begin{sidebox}
    [A=B]
end{sidebox}
end{sidebox}
end{document}