First I will say that the MODIS LUT updates will be coming more regularly. The MODIS-Aqua instrument degradation continues and is less predictable than we'd like.
The R2018 data are only considered "refined" through mid Oct 2017, as it takes time to evaluate the degradation and implement LUT updates to account for the changes seen.
We will be making more regular LUT changes and slowly advancing the "refined" data in time, but there will be a 60-90 day (or more) lag.
With that in mind, yes, your analysis may be premature. However, we're seeing the same thing:https://oceancolor.gsfc.nasa.gov/analysis/global/vt66_at155/plots/vt66_at155_olig_chl_ratio.png
(VT66 is effectively R2018 for VIIRS-SNPP, AT155 is for MODIS-Aqua - but does NOT include the latest LUT)
Welcome to the world of aging instruments - Aqua will have her 16th birthday this year and VIIRS hit 6 years late last year. We will continue to do what we can to track and correct for instrument degradation, but the problem is becoming increasingly difficult to tackle.
Oh, and, no, there were no algorithmic changes with this reprocessing, so the OC coefficients are not different.