My guess is that there is an algorithm built into the Braumeister to heat the water more to compensate for the loss of heat due to the "cold" grains...
That's also my theory. But I think the heat compensation (the algorithm) is activated at the "fill in malt" stage (i.e. when you start pouring the malt in the malt pipe - between the two stages "fill in malt" and "malt filled in?"). When pressing "Start" in the "malt filled in?" stage the unit enters the different phase stages.
I am more concerned with the loss of heat during the main mashing. When the internal thermostat is showing, say, 66C - my reading with the Thermopen off the top of the grain bed can give me 2-3 degrees lower.
Interesting how accurate your data is compared to mine. But like I said, the temperature drop is happening gradually through the whole malt bed - which is pretty thick (6kg)! So maybe 2-3 degrees Celsius isn't that bad after all. But still an issue you should be aware of (and find a solution to if possible).
I tend to max out the grain bill to just south of 6 kg - that certainly may have an impact on the circulation of the heated water, which leads to the temp drop.
A maxed out grain bill would certainly affect the circulation. And that would work counteractive to the heat compensation (the algorithm). The heat compensation probably works better at a grain bill of 4-5 kg.
I have the insulating jacket - but I often take it off during mashing due to the fact that the temperature often starts to drift upwards when using it.
I will give it a try next time (Sunday).
And a solution to the problem where the temperature rise during mash in could be to set the initial temperature (mash in/dough in) about 3 degrees Celsius lower than the first phase.