this is because the preprocessor was switched between 1.2019.5 and 1.2019.6.
Your example will work if you add brackets to LAYOUT_WITH_LEGEND()
Please refer to http://plantuml.com/en/preprocessing which states:
"When calling a legacy !definelong macro with no arguments, you do have to use parenthesis. You have to use my_own_definelong() because my_own_definelong without parenthesis is not recognized by the new preprocessor."
There is already a pull request in the underlaying repo: