I have a Magento 2.3.0 with js and css merging enabled. My first issue is with the fact that the docs say that this option only works in developer mode. This does not make any sense as the optimization should be for production environments.
My next question is when are the files under pub/static/_cache supposed to be created. From what I figure it is when they are first requested, which is not very logical as these file might get rather big and require many resources to be created.
The docs are wrong abt merge working only in dev mode.
Files under pub/static/_cache are created at request time. So make sure there are enough permissions on the directory.
See this thread of files for more info:
\Magento\Framework\View\Result\Layout::renderResult()
\Magento\Framework\View\Result\Page::render()
\Magento\Framework\View\Result\Page::assign()
\Magento\Framework\View\Page\Config\Renderer::renderHeadContent()
\Magento\Framework\View\Page\Config\Renderer::renderAssets()
\Magento\Framework\View\Page\Config\Renderer::renderAssetGroup()
\Magento\Framework\View\Asset\MergeStrategy\Checksum::merge()
Or this github issue: https://github.com/magento/magento2/issues/13225