Just read a rough story from someone flying Philly to Glasgow with American Airlines.
They had a connection through Heathrow and somehow the Heathrow to Glasgow flight plus parts of the return trip were cancelled in the system without any notice. They only found out while already in London even though they had boarding passes earlier.
They spent around 7 to 8 hours dealing with phone and chat support before getting rebooked, arriving in Glasgow more than 7 hours late. Then on the way home the Glasgow to Heathrow segment was still missing, so they ended up taking a train to London and paying for a hotel just to catch their flight back to the US.
Apparently AA support hasn’t really addressed the missing segment or compensation either, just generic replies so far.
Stuff like this is what makes mid-trip cancellations so stressful.