interesting.  have you checked by resetting this to zero in developer, and only making this change in the page block setting via the regular pages editor in the studio?
it sounds like you have conflcting settings between developer and pages modules, playing duelling banjos, and to make it more fun you also run OPCache, which has its own settings...
my suggestion:
developer cache 0 on all your blocks,
and tweak OPCache settings a bit,
but in pages module use cache setting on your pageblocks to be somewhat reasonable, (low time) because if new content is added it may not always pull the clean new copy and may serve a visitor a cached copy, and then somewhere the auto detection script will say "new content" but maybe not before they have browsed away already from that area...
this echos somewhat a similar problem i am having with my site, which is people are not used to a site that doesnt dynamically reload content in the given blocks/page section in every use scenario after they perform an action..