I have to use oyster grits in one of the 4 locations I make rum at in the course of a year or it will stall every time after very minimal fermentation - it's way too sweet and goopy to run and no discernible booze smell lifting the lid.
The "city" water (it's a first-world country, or used to be) at all locations is in the range pH 6.5-7.5,
with the most problematic location being the pH 7.5 location (yep!)
The "problem water" location is a residential room cupboard with heated brew mat (simple thermostat controlled mat under a plastic fermenter bin). I suspected problems like temp control, bacterial contamination, local sources of molasses being different, etc etc.
So, in an effort to get my head around the problem, I transported at great personal sphincter pain*, the entire ingredients including water in sealed jars to the nearest "OK for rum" location to rule out environmental factors.
Again, at the new location but using the transported pH 7.5 water, the problem remained - this alternative location had the benefit of very close PID temp control inside a professional brewing fridge. My work buddy there is a geeky ale-head with WiFi tilt hydros, CCTV in the lids, yeast slopes, laminar flow filtered air bench, the works.
He just about has an aneurism when he watches me mix up a batch of rum to start fermenting and insists I wear a facemask and gloves in there if he's doing his "yeast work".
However, adding a cup of oyster grits (chicken scratch grits, rinsed in water beforehand) prevents the problem and adding some after a stall (after a day or two delay) can usually restart these "problem water" batches.
So I tried the same for sugar washes, with similar results at both locations though it tended to be a slowdown rather than outright stalls for the sugar washes anyways. At least the car headlining, seats and floor didn't get coated in dunder for these.
So it's not JUST about start water pH, there has to be something more subtle going on at least in my case. Maybe some water has additional attributes other than the pH that can lead to pH crashes regardless of initial pH. Beerheads are incredibly picky about their water and whole cities and industries have been built on the local water characteristics.
Anyways, I can't really speak for low dynamic pH readings in the ferment causing rum flavor changes** (as opposed to starting water pH) because I could never get the bloody stuff to finish a ferment properly without oyster grits. But certainly, starting water pH is no absolute predictor of how low a ferment pH will crash later on, at least for me.
So since that fateful, sticky, smelly incudent transporting dunder in the car 400 miles, I have since always used some oyster grits in my rum ferments.
I don't "worry" about pH, I just nowadays always add oyster grits out of habit.
In the same way I don't "worry" about lack of rum flavor, but I always add a dead rat and several gobs of spit to the molasses out of habit
*if ever you drive any distance with dunder or dunder+muck, DON'T use a thin plastic flexible bottle/jug. Use something sturdy like rigid HDPE or a rugged glass jar. Ask me why I know.
** low pH rum ferments are alleged to have less "rummines", higher pH having more, so in theory oyster grits, oyster shells, marble, or similar could produce better rum flavors than without.