Hello Bisenberger,
yes I have found that normal prj export from Global Mapper is not read correctly by Metashape. This is probably caused by Global Mapper not using standard names for DATUM entry (prepending D_ to name).
However this can be solved by setting Include datum parameters and EPSG codes in WKT PRJ files in Global mapper Configurarion Export options as seen in first attachment...
You then get a prj with EPSG code that can be read by Metashape... see 2nd attachement where original prj (not read by MS) on right is compared with new format on left..