Hello @Vasilii,
Thanks for your feedback.
Unfortunately, Forest Admin does not support filtering on Smart Fields or Smart Relationships. Filters are directly used to create SQL queries, in order to run performant researches on your tables.
As described in this thread: Smart field filtering - #4 by lamatt_v you can define smart segments if you want to create a pre-defined set of searches.
In your case, why did you create a smart relationship, and not directly a hasOne association in your model? It seems to have a direct relation between a lead an a job.