Thanks for the confirm, Francois!
Indeed, the 1st Raman drop to a half has happened within but a few exposures, here is how it looks:

- title_Raman12_x6_y4.png (22.9 KiB) Viewed 8703 times
(generated by a trivial flux measure in IRAF (6804:6847, 7066:7100) splot/'e' tool, rectified to a stable 2nd order continuum)
The last 5 exposures had 20 minute, all others 10 minute.
Peter