lupdate does not work with project file using SUBDIRS target where the .subdir option is used

I'm having a main project file for a large project which uses the subdirs TEMPLATE to build the app itselöf plus njumerous plugins for it. Here is a part of the project file:

TEMPLATE = subdirs
SUBDIRS += application

1. also build the plugins
   SUBDIRS += plg_table
   plg_table.subdir = ./plugins/table

The project file for the "application" is in the application subdirectory, but the project file and code for the "table-plugin" is in ./plugins/table, so I use the .subdir option to change the directory of plg_table. Unfortunately this is not recognized by lupdate and lrelease. They work on the translations from application (and some other subdirs, like them which I left out) but do not find the directory "./plg_table" (error message), so the do not care for the .subdir option.