Although I have never seen any flare with my 21,25,35 Biogons and 50 Planar, it happened a few times with the 50 Sonnar and 28 Biogon. The only thing that can make these lenses flare, is the reflection of the metallic filter mount. Try screwing on an empty black 43mm filter ring, and do some shots with the light at oblique angles to see if this changes things.