hmm.. it seems this is the same issue as:
when putting the target_value in “Workout Step Fields” at 0, the custom target values are used
see FIT SDK documentation
Target Values vs Custom Target Values
Unless defining repeat steps, the target_value dynamic field typically refers to setting a target zone. These target zones represent target limits that have already been established through other means, such as in a settings file, through a user interface, or predefined on fitness equipment. If zones are predefined their numbering should start at 1 (since 0 is reserved to indicate a custom zone). The workout_step can then be used to set a target heart rate, power, or other zone value. In this case, the custom_target_value_high and custom_target_value_low fields should be set to 0.
If a specific target range is desired, the custom_target_low and custom_target_high fields may be used to set the upper and lower boundaries of the desired target range, and the target_value would be set to 0. Refer to the FIT SDK for specific field/zone values. When creating swim workouts, the target is used to indicate the swim stroke for the step.