Here's a link to Microsoft's Forest and Domain Functional Levels doc if you want all the details about DFL/FFL levels.
The important bit relating to your question is this (emphasis, mine):
Functional levels determine the available Active Directory Domain Services (AD DS) domain or forest capabilities. They also determine which Windows Server operating systems you can run on domain controllers in the domain or forest. However, functional levels do not affect which operating systems you can run on workstations and member servers that are joined to the domain or forest.
So in short, yes. You should have no problems adding a Server 2019 member server to a 2003 DFL/FFL domain/forest.