As I did not want to install flatpack I installed from source. Version 3.38 has a working search function. The newer versions do not work in bionic since they use a newer glib2-0 library (>2.66) than available for bionic.
Since the gmaps sources do not come with installation instructions here is how it can be installed on Ubuntu 18.04 - bionic. Gmaps does use Meson/Ninja for building the sources. Its not installed by default in bionic, so install it first
sudo apt install meson ninja-build
Get the sources here: https://download.gnome.org/sources/gnome-maps/3.38/gnome-maps-3.38.6.tar.xz. Download and extract tarball. For the example the folder with the extracted files was names "src".
First install all required dependencies (gjs-1.0, gee-0.8, folks, libgeocode-glib-1-0, champlain-0.12, rest-0.7)
sudo apt install libgjs-dev
sudo apt install libgee-0.8-dev
sudo apt install libfolks-dev
sudo apt install libgeocode-glib-dev
sudo apt install libchamplain-0.12-dev
sudo apt install librest-dev
To build and install enter the unpacked folder src. It should contain the file "meson.build"
cd src
meson --prefix=/usr builddir
ninja -C builddir
sudo ninja -C builddir install
The meson command should list all dependencies and show all with a green YES. I got one red NO which was "appstream-util", but the application still worked. (please comment if you know what it is, what the package name is and why its needed). Do not forget the "sudo" before the last install command.
The application is started from the command line with
gapplication launch org.gnome.Maps
You can put that line into a launcher in the panel. After installation the sources are not required any more and can be deleted.