drush migrate-import does not work with multiple groups

I'll classify this as support request rather than as bug report because I'm unsure, if this should work in general and if I'm deceving myself here.

I have defined three migration groups, that work as expected, when I call them one by one. However running them all, comma separating their names like the following command, does not work:

drush migrate-import --group=lw_workdays,lw_customers,lw_products

When I run this command, nothing happens: no (error) message, no import - it just ends quickly without doing anything.

When I look into the source code of the Drush commands, I see that being able to run multiple groups at once is not documented, although it should work theoretically, when you look at the function calls. And I'm pretty sure, that I've already done that successfully!

And yes, in the meantime I've updated migrate_tools, but there were only two new commits since then: one regarding the UI support, another one just updating readme file. So nothing really related...