Thanks for the suggestion. I just tried it again with 126.96.36.199. The problem still seems to be there; but I noticed that it does not occur when writing output from a QuickColor node with 'Input Auto Level' turned on; only when the output comes from other nodes like a Slope node or Multifractal.
I ran this test: create a blank project, add a Multifractal node, add an Output node attached to that. Add a Slope node attached to the Multifractal node, add an Output out of that. Add a QuickColor node, check 'Input Auto Level', attach to Multifractal, add an Output node from that. Set all Output nodes to PNG 8-bit. Set preview resolution to 0.5k, build resolution to 512. Do a 'Start Build' 4 times; the first time, select 'No' for 'Save current preview build?', select 'Yes' the next 3 times. Compare the output PNGs. Both the Slope output and the Multifractal output get darker and darker with each subsequent build; the QuickColor output does not. I attached the .tor file and subsequent outputs from the Slope node.