During the transition from slow fill of liquid hydrogen into the Space Launch System rocket’s core stage to fast fill operations, launch controllers saw a spike in the amount of hydrogen that is allowed to leak into the purge can, a housing covering the tail service mast umbilical’s quick disconnect, or mating interface with the rocket. Engineers started reverse flow of liquid hydrogen into the core stage and are currently troubleshooting.
Although a similar issue was identified in an earlier wet dress rehearsal, it may not necessarily be the same cause.