Hi @tirrr ,
Regarding the needs that you mentioned, I think you have some misunderstanding with the usage of the Sub Grid in Entity Form.
The Sub Gird mainly is used to display related records from other Entity corresponded to current Main Entity record through the "One-to-Many" relationship.
Based on the issue that you mentioned, I guess that you do not display related Entity records within the Sub Grid, instead, you display current Entity records in this Sub Grid component, right?
If you display your current Entity records in this Sub Grid, and want to filter the record based on the column value (e.g. Project) in the Entity form, I afraid that there is no direct way to achieve your needs.
Currently, within the "Sub Grid" component, you could not specify filter condition based on the fields within the Entity this "Sub Grid" component connected to.
If you would like this feature to be added in PowerApps, please submit an idea to PowerApps Ideas Forum:
https://powerusers.microsoft.com/t5/Power-Apps-Ideas/idb-p/PowerAppsIdeas
Regards,