How to break line in a theorem when using lists?

I have defined my theorem style as:

newtheoremstyle{ejemplo} % Style name
{2ex} % Above space
{2ex} % Below space
{normalfont} % Body font
{} % Indent amount
{bfseries} % Head font
{} % Post head punctuation
{newline} % Post head space
{thmname{#1}thmnumber{ #2}:thmnote{ [#3]}} % Head spec

However when I use it in combination with lists like this:

begin{obs}
begin{enumerate}
item Bla bla bla 
item Bla bla bla
end{enumerate}
end{obs}

I do not get the new line. The problem is resolved if I add something before the lists:

begin{obs}
This fixes it
begin{enumerate}
item Bla bla bla 
item Bla bla bla
end{enumerate}
end{obs}

How can I fix this?

---

Edit: Sorry. Here’s the minimal working example:

documentclass[12pt, letterpaper]{memoir}

% Packages
usepackage[T1]{fontenc}
usepackage[utf8]{inputenc}
usepackage[spanish]{babel}
usepackage{mathtools}
usepackage{amssymb, amsfonts, amsthm}

newtheoremstyle{ejemplo} % Style name
{2ex} % Above space
{2ex} % Below space
{normalfont} % Body font
{} % Indent amount
{bfseries} % Head font
{} % Post head punctuation
{newline} % Post head space
{thmname{#1}thmnumber{ #2}:thmnote{ [#3]}} % Head spec

theoremstyle{ejemplo}
newtheorem*{obs}{Observación}


begin{document}

begin{obs}
begin{enumerate}
item Bla bla bla 
item Bla bla bla
end{enumerate}
end{obs}

begin{obs}
This fixes it
begin{enumerate}
item Bla bla bla 
item Bla bla bla
end{enumerate}
end{obs}

end{document}

documentclass[12pt, letterpaper]{memoir}

% Packages
usepackage[T1]{fontenc}
usepackage[spanish]{babel}
usepackage{mathtools}
usepackage{amssymb, amsthm}
usepackage{enumitem}
newlist{thmenum}{enumerate}{1}
setlist[thmenum]{label=arabic*., itemsep=2pt, before=leavevmode}

newtheoremstyle{ejemplo} % Style name
{2ex} % Above space
{2ex} % Below space
{normalfont} % Body font
{} % Indent amount
{bfseries} % Head font
{} % Post head punctuation
{1em} % Post head space
{thmname{#1}thmnumber{ #2}:thmnote{ [#3]}} % Head spec

theoremstyle{ejemplo}
newtheorem*{obs}{Observación}

begin{document}

begin{obs}
begin{thmenum}
item Bla bla bla
item Bla bla bla
end{thmenum}
end{obs}

begin{obs}
This fixes it
begin{enumerate}
item Bla bla bla
item Bla bla bla
end{enumerate}
end{obs}

end{document}