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LM431: Higher ripple on AC/DC power supply output?

Part Number: LM431
Other Parts Discussed in Thread: TL431


Customer is using the LM431 (LM431BIM3/NOPB) on an AC/DC (PFC flyback: 1Ph 230V AC to 24V DC 5A with PFC voltage: 390V).
The LM431 is used for the feedback sensing on the secondary side.
With the last LM431 lot the ripple (around 600mVp-p) on the DC Power supply voltage output goes outside of the acceptable range.
If LM431 of this lot are replaced on the same PCB with a LM431 from older lot or TL431 then the ripple goes back to an acceptable value.

I can provide offline:
  -T&R sticker of the incriminated lot
  -picture of marking of a "good" and "not Good" LM431.

Any feedback you can provide?
Could you please provide some feedback on the schematics?

Thanks in advance,