Biblatex not printing umlauts

Question

I have a bibliography entry with a german umlaut in the name. In the .bib file the character is correctly escaped:

author={B{\"o}ttcher}

The corresponding entry in the .bbl file no longer contains the escape sequence but the ASCII character ö:

\entry{Bott02a}{inproceedings}{}
  \name{author}{2}{}{%
    {{hash=471f6641bcc35e7859446a7099bd375c}{%
       family={Böttcher}
...

In my bibliography then the entry is printed without the umlaut as "Bttcher". Manually replacing the ASCII ö with {\"o} in the .bbl file fixes that.

What do I have to do to get the ö in the output?

Notes:

• I use lualatex and biber
• I use a custom bibliography style based on "standard"
• I do not use any encoding packages explicitly
• I have tried configuring biblatex with texencoding and safeinputenc without success

Edit:

Here's a minimal working example

test.tex:

\documentclass{book}
\usepackage{biblatex}
\addbibresource{test.bib}
\begin{document}
  \cite{Bott02a}
  \printbibliography
\end{document}

test.bib

@inproceedings{Bott02a,
  title={title},
  author={B{\"o}ttcher}
}

Run with lualatex test; biber test; lualatex test; open test.pdf

Update:

If you happen to be unlucky (like me) you'll be using mylatexformat to precompile your preamble. In that case fontspec will give you hell. Here's a trick that makes it possible to use fontspec within the preamble (taken from Precompile header with xelatex):

\documentclass{...}
...
\RequirePackage{etoolbox}
\AtEndPreamble{
  \usepackage{fontspec}
}
...
\endofdump

Note that fontspec will not be compiled into the format and thus loading time for fontspec cannot be improve by precompilation.