PartySausage 60-70 & KH of 40. The last drop test I did on the water from the steam boiler a couple of weeks ago showed hardly any increase in GH or KH @ 25degC from the water I’ve been putting in.
I removed the shower screen & also drew some water from the brew boiler, which set to 94 DegC & there weren’t any flecks in that; so it does seem isolated to the stream boiler, which is set to 129 DegC
At this alkalinity and temp the threshold for hardness before you see scale is about 45mg/l. It’s Magnesium carbonate scale and that it is more soluble than calcium isn’t of particular significance….it is still going to form scale, sure comparatively more will be dissolved when the water cools compared to calcium but that’s it.
Given you can’t have more than 45mg/l hardness in the water with 40mg/l alkalinity after heating it to 129c, I’m surprised your drop test shows 60gh and 40kh after letting it cool down….