Geometries saved in BEAM-DIMAP cannot be restored. The correct class for restoring the information is not loaded.
This is related to the forum discussions:
SNAP 8 - cannot save vectors in dim
Impot csv file - s3tbx / Problem Reports - STEP Forum (esa.int)
Save Vector training samples - SNAP-8 - s2tbx - STEP Forum (esa.int)
This has been resolved in commit
fix is included in v8.0.2