One thing I could do is add some bacteria and see if I get the same results. I have several bottles of bacteria in a bottle I bought to see what was in them seen here:
Starting at post 179
https://www.reef2reef.com/threads/our-tanks-biology-up-close.239838/page-9
Since I will be adding it directly I should see the drop fairly quickly and not over night like with the marine pure events seen in the above chart. I would use the EcoBalance as thats the one I saw the most active and live bacteria in.
Randy's tests with Aluminum showed an immediate drop in ph and a slow rise over a period of a few hours. Your observations appear different. Still don't see how it could be bacteria though!
What did you do with the block while it was out of water? How long was it out of the water? Did you rinse with tank water before you put it back in?