OK I think I understand what's going on and it's mostly (but not all) my fault. I've upgraded to 1.19 RC1 BTW.
What's throwing me is that clicking on any tool switches them all from active to standby, (but that's not really true). That is to say, the column next to "Tool" is "Heater" and it's that column which toggles from "Active" to "Standby" whenever I click on any tool. So under "Tools" I have Tool 0, Tool 1 Tool 2 etc and under "Heater", next to each tool I have Heater1. Heater 1, Heater 1 all the way down (because they all use the same heater). So it just looks like clicking on any tool, is toggling the state from Active to Standby for all tools because the heater next to the tool changes state. This is what was confusing me because I can have a heater showing active next to a tool that isn't actually active. When I look closely, the little underline for the tool is still present which is what actually denotes which tool is truly active. I have to ignore the word "active" and look for the little underline instead.
So now I understand and it was largely me being thick (nothing new there) but I'll put a DWC request to have the active tool denoted more prominently, like in a different colour or bold, rather then the faint, hard to spot, underline.