This is an interesting finding and it is possible. There is no telling if the assumption that Bru'n Water uses (malt acidity is generally proportional to color rating) ALWAYS holds true. I can say that appears to be true for the vast majority of base and crystal malts, so that should be reassuring. But I don't know that Melanoidin malt conforms with that assumption.
As pointed out above, I would expect that with the typically low percentage of Melanoidin malt in a grist would mean that it shouldn't have a lot of effect on the overall wort pH. If there was a large shift in the wort pH, I would be more inclined to look at the base malt. Do recall that we have observed that some base malts tend to be more acidic than their color rating implies. Rahr malts tend to show this response and I've recommended that you trick the program into duplicating that higher acidity by adding about 3L to the reported Rahr base malt acidity.
A similar issue with malt acidity was brought to my attention by a British brewer several months ago. He reported that Crisp Amber malt did not perform as expected with Bru'n Water. The maltster lists their amber malt as a Roast malt. In that brewer's observations, he found that the mash pH prediction was off when the Amber malt was assumed to be a Crystal malt, but was almost perfect when set to Roast malt in Bru'n Water. With a color rating of only about 30L, this certainly doesn't seem to be a Roast malt. But the maltster's categorization and the apparent acidity suggest that it is. For Bru'n Water users, you have probably noted that I have suggested that Roasted malts are those types that have a color rating of over 180L. It appears that I am wrong again! So we have another case where malt color may not be the best indicator for acidity.
We will keep working at it!
thanks so much for the info. i did switch from regular maris otter to Warminster Floor Malted Maris Otter, and that's about 88% of the grist. im targeting around 5.25-5.3, so when it was getting 5.08-5.16 i wondered what happened.
I did originally list melanoiden as crystal, and since changed it to roast malt. bru'n water projects 5.3.
the only other thing i can think off as partial contributor is my RO water not quite aligning with the RO defaults. perhaps with TDS of 12-13, PH 6.2 that there's not much of any buffering capacity and my water is more similar to distilled. not sure but just a thought.
edit: i just pulled this info from the manufacturers website on floor malted maris otter:
Extract Colour
As Is Dry 450g 515ml
Maris Otter Premium Pale Ale 300 309 5.3 4.5
so changing the melanoiden to roast, and changing the maris otter color from 3.5 to 5, and changing water profile from RO to distilled moves the projected PH to PH 5.27. my actual on one of the batches came in at PH 5.16, variance .11. interesting - might be on to something with these changes and theories.