@CorentinLabelle, thank you for sharing the data .
The issue is now solved at: 6d8c21d
Details FYI: At exporting the valid and invalid examples, the EDF files were created with UTF-8 encoding. In the EDF file, annotations (events) are distributed across the multiple records in the EDF file (number of records is given by the recordings length). The invalid example had less events, then some records (including the first one) did not contain annotations, then at reopening the EDF file (to continue writing), without an explicit encoding Matlab's fopen() detected it as UTF16-LE and wrote with that encoding => Bug. Now, UTF-8 encoding is explicit in Brainstorm for writing EDF files