Join nodes do not properly propogate updates

[Bruno-TT]

Hi, just a small one - if we have something that looks like

prompt node -> join node -> prompt node, rerunning the first prompt node should "invalidate" the second prompt node (i.e. it should display that we need to rerun it), whereas atm the second join node keeps the tick above it and doesn't say it needs to be rerun. Thanks very much :)