It seems to me that whatever is causing the low GFR involves a change made after the 112 reading.
Some synthetic B12 supplements contain cyanocobalamin or aluminum. Our kidneys don't process either very well so you could have a build up affecting your kidneys. High doses of B12 have been noted in association with faster decline of kidney function. Methylcobalamin is natural, but it is also more bioavailable, so you might be absorbing a higher dose.
You might change brands and/or lower the dose for October before the November draw.