I did a lot of research when my extension's frontend plugin (TYPO3 7.6.4) refused to use the 'pages' field of the plugin ("Record Storage Page"), so I would like to share my findings:
My extension's name is 'tx_dhsnews', my plugin's name is 'infobox'
setRespectStoragePage must be set to true (default): $query->setRespectStoragePage(TRUE)
In the typoscript-setup, the plugin-specific storagePid plugin.tx_dhsnews_infobox.persistence.storagePid MUST NOT be present at all! Not event with an empty value! Else the 'pages'-field will not be respected!
That's all. The Extensions Builder just created a typoscript-setup with the storagePid for the specific plugin 'infobox' set to nothing. That resulted in the plugin not respecting the 'pages' - field.
It's no problem to set the storagePid on extension-level (e.g. 'tx_dhsnews..persistence.storagePid'), the value will be merged with the value(s) given in 'pages' ("Record Storage Page"), but as soon the plugin-specific tx_[extension]_[plugin].persistence.storagePid exists in the typoscript, it will overrule everything else!
Hope this will help somebody to save some time + nerves