you might want to update the advanced settings for issue links according to the instruction in the documentation. You should ue parameter multiple_values = true and remove parameter split_by = ",". Please follow the instructions and examples on importing issue links: Import issue links.
Note that you should perform double data import for “Issue Links” field after the changes to ensure correct outcome.
In the import option, deselect the custom field “Issue Links” from data import and import data. This action will clear the previous data and data structures.
Wait for data import to complete.
In the import option, select the custom field “Issue Links” for data import and import data for the second time.
When the links ar correctly imported, there are several options for counting linked issues. Please see those Community posts on several approaches with claclautiona or JavaScript calcauted measures: