This issue manifests itself this way: if some WHERE condition (COL3 >= 0 in our case) is evaluated to NULL/UNKNOWN for the prior (skipped) row, then all blobs are set to NULLs in the next row (unless explicitly set to something else by the SET clause). This happens only for BLOB fields and only if OLD and NEW records have different formats (i.e. after ALTER TABLE).
The bug became visible after fixing CORE5600 (thus v3.0.3 and later versions are affected), but the issue itself seems to be old. We were just lucky that the "buggy" code path was never used before.