That sounds like a reasonable possible explanation, but I'm afraid it's wrong, as is my whole idea that carrageenan is the cause of what happens to my readings. I finally did find some reasearch on this, here: https://onlinelibrary.wiley.com/doi/pdf/10.1002/j.2050-0416.1995.tb00860.x . On p. 188: Preliminary experiments also established that addition of copper finings to wort did not significantly alter cold wort pH(TableI 1).
Searching the net, I found a few others posting questions about the same problem. No good answers, though most suggesting that there might be something wrong with the pH meter. I'm sure that's not the case here. I use the meter several times throughout the process, and when it doesn't give me any readings outside of what I would expect until that last one, I don't think there's any reason to suspect that it is something wrong with it.
I'm really not sure what to think. If this is a real problem, it's strange that it isn't recognized. But I guess most people don't take readings post boil. If you take a preboil reading, you should expect around 0,2 below that when the boil is done, according to theory. So why bother? Then it also seems to be a widespread belief that the yeast will take the pH down to the right level anyway. And if you read Palmer - which I generally think is a good idea - he tells you that the mash pH will set up the pH for the rest of the process.
I don't think any of these assumptions are correct. Monitoring pH as the brew progresses has taught me it's not at all that straightforward.