By default, the qt_add_translations command creates the .ts files in the CMAKE_CURRENT_SOURCE_DIR, i.e. the project's source directory. This location can be changed by passing a different directory to the TS_FILE_DIR argument.

When specifying the plurals-only .ts file for the source language with PLURALS_TS_FILE, if the TS_FILE_DIR argument is set, then the qt_add_translations() command ignores the specified directory and generates the plurals .ts file in the project's source directory. For example,

qt_add_translations(
  apptranslations_plurals
  TS_FILE_DIR
  translations
  PLURALS_TS_FILE # Ignores the set TS_FILE_DIR directory
  translations_plurals_en.ts)

In order to generate the plurals .ts file in the same directory set by TS_FILE_DIR, the directory needs to be specified explicitly in the PLURALS_TS_FILE argument. For example,

qt_add_translations(
  apptranslations_plurals
  TS_FILE_DIR
  translations
  PLURALS_TS_FILE  
  translations/translations_plurals_en.ts)

To reproduce issue, build attached project with update_translations.