Dynamic dropdown list values are not refreshed correctly

I’m trying to make a service dialog that has two drop-down lists:

Whenever value is chosen in 1st drop-down list, values in 2nd drop-down list should be refreshed, however after refresh they are not updated. Clicking the refresh button does not update them either.
The only option to update them is selecting any position in 2nd drop-down list and then reopening dialog.

My configuration of 1st drop-down is as below:

2nd drop-down configuration:

2nd drop-down populate method:

Is there any way to make the values from 2nd drop-down list refreshing after selecting option from 1st list ?