Beam weapons - including the Tachyon Lance - do "soft flux". Soft flux - from incoming beam weapons, your own weapons firing, and your shields' maintenance costs - can be dissipated while shields are up, while hard flux - from incoming non-beam hits - can only be dissipated while shields are down. In other words, if your flux dissipation is high enough, you can simply ignore those lance hits.
(Well, okay, if your hard flux level is high, the Tachyon Lance in particular has a shield-piercing EMP effect. But that only triggers if your hard flux level is high.)
Between that and the damage reduction for playing on easy mode, this seems like it's probably not a bug.