I have two questions:
(1) Did you set recovery_target_timeline='latest' in both master and slave?
Yes....but it's in recovery.conf so it only really applies to whichever server is currently the slave...
(2) Did you make any changes after promote the slave to be master?
Yes, some....however I'm not sure I've done enough changes to have the slave rotate+archive an xlog before bringing the old master back up as a slave....I would assume that wouldn't make a difference but it's something to test I guess....