Yes it IS weird, and yes it does seem like a kind of resonance involving my ear and ear canal structure. The open PX100II doesn't give even a hint of this problem.
The particular track I've keyed on today is the first movement of Schumann's Symphony No. 1 in B-flat major, Netherlands Radio Chamber Philharmonic, Michael Schonwand conducting (Spotify). The opening movement has a sudden flourish of horns, woodwinds and strings, and even with DT250-250 and PX100II there is not much sense of definition of individual instruments. With the DT1350 I hear a glaring mass of sound, and maybe with some imagination can tell there are horns and strings. In addition, it's smaller-scaled and less enjoyable; turning the volume up helps that bud worsens the glare.