Flow doesn't process multiple value in custom list

Here is one workaround:

1. Create variable called, for example, test1;

2. Create variable called, for example, test2;

3. Set test1 variable to your multiple choice field value (it will be automatically put in "Apply to each"):

   3.1. Name = test1 variable + delimiter (in my case delimiter is coma);

   3.2. Value = yuor multiple choice field.

4. Right after setting test1 variable add action "Append to string variable":

   4.1. Name = test2 variable;

   4.2. Value = test1 variable.

5) Put output of "Append to string variable" wherever you need.

See screenshot with example.