Easy to reproduce with one of Qt examples that uses static plugins. Edit examples/tools/plugandpaintplugins/basictools/basictools.pro and add:
CONFIG += create_prl
then build it - no .prl file created.
Now remove "plugin" from CONFIG list and build again - .prl file is created. But without "plugin" option the library is useless as a plugin because it is missing important symbols e.g qt_plugin_instance_<plugin_name>().