I tried to implement what @premkumarravi proposed. The midStep part worked very well. The return section was causing me problem, since values line didn't accepted the [fullKey] argument as valid.

Inspired from his proposal, i finaly did that for the return statement

    SUMMARIZE(
        MidStep,
        [date],
        "liste mandats", 
            CONCATENATEX(
                 filter(MidStep, [date] = earlier([date])),
                 [fullKey], " ** "
            )
    )