id is the primary key for
pstx_project then you definitely don't need
pstx_project_company_id in your many to many breakdown table. As you say, these tables just need their related tables primary keys (in which it's composite will become it's own primary key), and a separate foreign key against each referenced table (which might be composite also, depending on the amount of columns from the referenced table's primary key), with the addition of other optional columns you might want to add.
company_id was part of the primary key of
pstx_project in some point, then it would make sense to include it in your many to many table. Maybe you edited the primary key at some point?
pstx_project_company_id from your many to many table as it will bring you problems with consistency, as it would be redundant. Also make sure that
user_has_pstx_project is a primary key so it enforces non-duplicates.
As a side note, I also note a loop in your design: there are 2 ways to get to a company from a user. While loops are possible in some scenarios, please review if it's correct for your case as you might have another redundant link between user and company.