The After Effects exporter has several bugs. Use the Adobe Media Encoder exporter for this format (which is what Premiere Pro uses).
This is a bug in After Effects CC 2015. We recommend you do not export MXF OP1a directly from After Effects. The connection to the MXF OP1a exporter is fragile.
Instead, you should render your composition to MXF OP1a using Adobe Media Encoder. Or, a common workflow is to render an intermediate file from After Effects and use a watch folder in AME to automatically to MXF OP1a.