Custom pivot class doesn't respect $fillable when not in production

[ChrisThompsonTLDR]

• Laravel Version: 8.56.0
• PHP Version: 7.4.23
• Database Driver & Version: MySQL 8.0.26

Description:

When using a custom pivot class the $fillable array is only honored when the application is not in "production."

Steps To Reproduce:

1. set your app to non-production
2. create 2 models that pivot through a custom pivot class
3. give the pivot class a limited array of fields that are $fillable
4. Write to pivot fields that are not set in $fillable
5. check db to see that data was written
6. flip your application to production
7. repeat 4 and 5
8. check db to see data was not written