npm run langium:generate does not generate optional field in base class but does in derived class, resulting in TypeScript error

[steve-hickman-epistimis]

In the example below, the test Langium grammar contains, among other things, the following:

ConceptualEntity infers ConceptualEntity:
    ConceptualAssociation | 'centity' name=ID  (description=STRING )? (':' specializes=[ConceptualEntity:QN ] )? '{' composition+=ConceptualComposition * ('basis:' '[' basisEntity+=[ConceptualBasisEntity:QN ]+ ']' )? '};'  
;


ConceptualAssociation infers ConceptualAssociation:
    'cassoc' name=ID  (description=STRING )? (':' specializes=[ConceptualEntity:QN ] )? '{' composition+=ConceptualComposition * ('basis:' '[' ConceptualBasisEntity+=[ConceptualBasisEntity:QN ]+ ']' )? ('participants:' '[' participant+=ConceptualParticipant + ']' )? '};'  
;

Note that the field specializes is optional in both ConceptualEntity and ConceptualAssociation.

The generated ast.ts contains the following exports:

export interface ConceptualEntity extends AstNode {
    readonly $type: 'ConceptualAssociation' | 'ConceptualEntity';
    basisEntity: Array<Reference<ConceptualBasisEntity>>
    composition: Array<ConceptualComposition>
    description?: string
    name: string
    specializes: Reference<ConceptualEntity>
}

export interface ConceptualAssociation extends ConceptualEntity {
    readonly $type: 'ConceptualAssociation';
    composition: Array<ConceptualComposition>
    ConceptualBasisEntity: Array<Reference<ConceptualBasisEntity>>
    description?: string
    name: string
    participant: Array<ConceptualParticipant>
    specializes?: Reference<ConceptualEntity>
}

Note that the specializes field in the exported interface ConceptualEntity is not optional but is optional in the exported ConceptualAssociation interface. This inconsistency causes a Typescript error.

Langium version:
Package name:

Steps To Reproduce

1. yo langium to create a new Langium project (see screenshot)
   
2. Substituting the Langium grammars in the linked files for the default grammar generated in the prior step
3. npm run langium:generate to generate, among other things, the ast.ts content.

Link to code example:
Terminals.txt
test.txt
ast.ts.txt
Note: All file extensions changed to .txt because GitHub doesn't support .langium extensions
Find linked above 2 Langium grammar files (Terminals.txt and test.txt). Also find linked above ast.ts.txt, the ast.ts file generated

The current behavior

Base class specializes field is not optional

The expected behavior

Base class specializes field is optional