iOS app - .xls files not opened with Collabora online

when I try to open an old MS Excel .XLS file on the iPhone, it is opened within the viewer only and it’s not possible to edit it. .doc files are opened within Collabora online and can be edited. In the web-browser both are opened with Collabora.

I’m running NC 18 with Collabora online developement edition 4.2.4 in docker.

Is this a miss-configuration of my app or a normal behavior?