The complicated script removes the 2 "Inifinity"s and "The Berlin Orchestra"s.
As there are no multi-value fields in the sample file, I honestly do not really understand why you only got the first part from the field composer and artist - you can see in my screenshot that with identical data as in your screenshot I get the full list as expected
... and I adapted the script so that it works with ; ... it should not be necessary now to replace all ; with /
There are no multi-valued fields because I deleted them manually, which is what I wrote... but how can I do this automatically for thousands of files?
To merge multi-value fields, either use $meta_sep() in scripting to keep the fields as they are or use an action of the type "Merge duplicate fields"
So, if you need to combine the values of two fields into a third one, for example, from two fields:
Field1 — Value2; Value1; Value3; Value4; Value5
Field2 — Value6; Value7; Value1
get a third one containing all the values from these two fields, but excluding repetitions:
Field3 — Value1; Value2; Value3; Value4; Value5; Value6; Value7
in the file context menu, select «Convert» > Tag - tag. Fill in the sample (see screenshots), using the following script:
$reverse($regexp($regexp(';'$reverse($meta_sep(FIELD1,; ); $meta_sep(FIELD2,; )),'\s*(;[^;]+)(?= (\s*\1|;.*?\1))',),'^\s*;+|;+\s*$',))
and replacing the field names in it with your own.
Thank you very much @ohrenkino for your help!!
I didn't find a button to edit the post, so I'll add it as a response. While using the script, I found an error that created extra spaces. I was able to fix the script myself. Here's the corrected script:
$reverse($regexp($regexp([';']$reverse([$meta_sep(FIELD1,; ); ][$meta_sep(FIELD2,; )]),'\s*(;[^;]+)(?= (\s*\1|;.*?\1))',),'^\s*;+|;+\s*$',))
Read the previous post for details!
(I had to sweat a bit more, but now it seems to be working correctly:))
It's the pencil in the tools below the post:

