Your class look a bit suspicious to me.

JobFilterContentApiModelFilter should implement IContentApiModelFilter and be injected, eg

// NO ATTRIBUTES HERE
public class JobFilterContentApiModelFilter : IContentApiModelFilter
{
	private readonly IContentLoader _contentLoader;
	private readonly ContentConvertingService _contentConvertingService; // or possibly IContentConverterResolver?
	private readonly IJobPageFinder _jobPageFinder;
	private readonly IServiceProvider _services;

	public JobFilterContentApiModelFilter(
		IContentLoader contentLoader, 
		IJobPageFinder jobPageFinder,
		ContentConvertingService contentConvertingService
		IServiceProvider services)
	{
		_contentLoader = contentLoader;
		_contentConvertingService = contentConvertingService;
		_jobPageFinder = jobPageFinder;
		_services = services;
	}
}

Inject in startup

services.AddSingleton<IContentApiModelFilter, JobFilterContentApiModelFilter>();

And work your way from there.

I found the answer myself, thanks @Eric for your assistance. The root issue is that there is a circular dependency between ContentApiModelFilter and ContentConvertingService. The latter is using the former to filter the contentApiModel at the end. But my implementation needs the ContentConvertingService to get the models of each JobPage.

• So first ContentConvertingService must be lazy to avoid a dependency injection issue which does not allow circular dependencies. 
• The next challenge(my question here) was that there was an infinite loop since my filter calls the converting-service that uses the filter that uses the converting-service ...
  • The solution is to create a new ConverterContext, one for each JobPage. Look at GetAllJobPageModels:

[ServiceConfiguration(typeof(IContentApiModelFilter), Lifecycle = ServiceInstanceScope.Singleton)]
public class JobFilterContentApiModelFilter : IContentApiModelFilter
{
	#region constants

	private const string _PROPERTYNAME_JOBS = "Jobs";
	private const string _EXPAND_ALL_PROPERTIES = "*";

	#endregion

	#region fields

	private readonly IContentLoader _contentLoader;
	private readonly Lazy<ContentConvertingService> _lazyContentConvertingService;
	private readonly IJobPageFinder _jobPageFinder;

	#endregion

	#region constructor

	/// <summary>
	/// Creates a new object of type <see cref="JobFilterContentApiModelFilter"/>.
	/// Is called automatically by the system.
	/// </summary>
	/// <param name="contentLoader">The <see cref="IContentLoader"/> instance to load content information</param>
	/// <param name="jobPageFinder">The <see cref="IJobPageFinder"/> instance to resolve all jobs of the site's job-portal</param>
	/// <param name="services">Service to get an instance of <see cref="Lazy&lt;ContentConvertingService&gt;"/> to resolve all jobs of the site's job-portal</param>
	public JobFilterContentApiModelFilter(
		IContentLoader contentLoader,
		IJobPageFinder jobPageFinder,
		IServiceProvider services)
	{
		_contentLoader = contentLoader;
		_lazyContentConvertingService =
			new Lazy<ContentConvertingService>(services.GetRequiredService<ContentConvertingService>);
		_jobPageFinder = jobPageFinder;
	}

	#endregion

	#region properties

	Type IContentApiModelFilter.HandledContentApiModel => typeof(ContentApiModel);

	#endregion

	#region Filter-method

	/// <summary>
	/// The main filter method. In this method, we check if the content is a <see cref="JobFilterBlock"/>.
	/// In that case all existing job pages are added to the contentApiModel.
	/// The filtering is done on client-side.
	/// </summary>
	/// <param name="contentApiModel">
	/// The current <see cref="ContentApiModel"/> we'll be adding the job-pages to
	/// </param>
	/// <param name="converterContext">The converter context with information about the current content</param>
	public void Filter(ContentApiModel contentApiModel, ConverterContext converterContext)
	{
		if (!_contentLoader.TryGet(converterContext.ContentReference, out IContent? content)
			|| content is not JobFilterBlock jobFilter)
		{
			return;
		}


		// this is a JobFilterBlock, add all job-pages models to the contentApiModel
		List<object> allJobPageModels = GetAllJobPageModels(converterContext);

		if (!allJobPageModels.Any())
		{
			return;
		}

		contentApiModel.Properties[_PROPERTYNAME_JOBS] = allJobPageModels;
	}

	#endregion


	#region private methods

	/// <param name="converterContext"></param>
	/// <returns>A List containing all job-pages' api-models</returns>
	private List<object> GetAllJobPageModels(ConverterContext converterContext)
	{
		IList<JobPage> allJobs = _jobPageFinder.GetAllJobPages();
		List<object> jobModels = new(allJobs.Count);
		jobModels.AddRange(allJobs.Select(GetJobApiModel));
		return jobModels;

		object GetJobApiModel(JobPage job)
		{
			return _lazyContentConvertingService.Value.Convert(job, GetJobContext(job));
		}

		ConverterContext GetJobContext(JobPage job)
		{
			return new ConverterContext(
				job.ContentLink,
				converterContext.Language,
				converterContext.Options,
				converterContext.ContextMode,
				null,
				_EXPAND_ALL_PROPERTIES,
				converterContext.ExcludePersonalizedContent
			);
		}
	}

	#endregion
}