In related lists, if you use the "is same" operator on parent.<fields> in conditions and click the Edit button, the slushbucket does not show correctly filtered records.
Navigate to Incident > Open and select any any incident record.
Configure Incident > Parent related list.
On the related list configure the following condition:
[company] [is same] [Parent.company].
Click the Edit button on the related list.
Note that the filter does not work and the slushbucket does not show the correct records or any records.
Change the Edit Ui action for the specific related list.
If it is currently using the OOB global UI action, copy that UI action and make it specific to this related list.
Add the following line to the script to add a filter:
uri.set('sysparm_query', 'company=' + parent.company);