update after some tinkering:
-ive now mashed with the melanoiden with similar quantities in other recipes, and no abnormal PH variance vs. Bru'n water.
- ive mashed with floor malted maris otter with no melanoiden, and no abnormal PH variance vs. Bru'n water.
- i mashed today with regular maris otter crisp, and similar quantities of melanoiden in previous recipe with the floor malted maris otter, and no abnormal PH variance vs Bru' n water.
So, it appears that the combination of the melanoiden and the floor malted maris otter make for a more acidic mash that is not going to calculate accurately in bru'n water.