Ah yes, i see the issue. You can do this a in a bit hacky way, changing:
{{myarr + query_related_filters.join(',', 'fts_filter_prop_fgn')}}
to
{{myarr + query_related_filters.join(',', 'fts_filter_prop_fgn') + ','}}
This way there will always be an ,
at the end, but i believe it should be working fine even with it.