HasOne not saving when using ->relationship() in a Group/Section/Fieldset

Repeater::make("regNumber")
    ->relationship('registrationNumber')
    ->saveRelationshipsWhenHidden()
    ->maxItems(1)
    ->hiddenLabel()
    ->addable(false)
    ->deletable(false)
    ->schema([
        TextInput::make('number')->label("Registration Number")
            ->live()
            ->columnSpanFull()
            ->inlineLabel()
            ->required(),
    ])
    ->columns(4)
    ->columnSpan(1),

Section::make()
    ->relationship('registrationNumber')
    ->schema([
        TextInput::make('number')->label("Registration Number")
            ->live()
            ->columnSpanFull()
            ->inlineLabel()
            ->required(),
    ])
    ->columns(4)
    ->columnSpan(1),