If the ccache class is include (meta/classes/ccache.bbclass), it invalidates most (all?) checksums/signatures, requiring full tasks execution. The idea is to avoid this and when introduce it into the configuration, no task re-execution should be done (as with buildhistory, for example).
I *think* I solved this, patch on the list.
Various fixes merged as of oe-core ffeca7dcc1abe82cad8374a31bf72b36a472fa1b.