Join nodes do not properly propogate updates

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 :)

Reproduce the prompt node → join node → prompt node flow described in the issue, then inspect the node update and invalidation entry points. Rerun the first prompt node and verify that the downstream prompt node loses its tick and indicates that it needs to be rerun.

Written by the indexing model from the issue text.