TaskCompletionSource restarts api call

Question

So I've ASP .NET application that exposes two endpoints. One for starting a continuous Task and other to time it out
My controller looks something like this

public class SomeController: ControllerBase
{
    private readonly ISomeService _service;
           
    public SomeController(ISomeService service)
    {
        _service= service;
    }
    
    [HttpPost, Route("service/stop")]
    public ActionResult StopService()
    {
        _service.StopService();
        return Ok();
    }
    
    [HttpPost, Route("service/start")]
    public async Task<ActionResult> StartService()
    {
        int result = await _service.Start();
    
        return Ok(new {Result = result});
    }
}   

and the service looks like

public class SomeService : ISomeService
{
    private static TaskCompletionSource<bool> tcs;

    private readonly IInternalService _internalService;

    public SomeService(IInternalService internalService) => _internalService = internalService;

    public void StopService()
    {
        tcs.TrySetException(new TimeoutException("Operation timed out") );
    }

    public Task<int> StartService()
    {
        tcs = new TaskCompletionSource<bool>();

        EventHandler<EventArguments> eventHandler = (sender, e) => EventHandler(sender, e, parameter1, parameter2);
        try
        {
            _internalService.Event += eventHandler;

            while (!(eventResult = (await tcs.Task))) ;
        }
        catch (Exception ex)
        {
            _logger.LogError($"Event failed because of {ex.Message}");
            throw;
        }
        finally
        {
            _internalService.Event -= eventHandler;
        }
    }

    private void EventHandler(object sender, EventArguments e, string parameter1, bool parameter2)
    {
            if (some condition)
            {
                tcs.TrySetException(new Exception("Event handling failed because condition was not met");
                return;
            }

            try
            {
                _internalService.SomeOperation(parameter2);
                tcs.SetResult(true);
            }
            catch (Exception ex)
            {
                tcs.TrySetException(ex);
                return;
            }
    }
}

When the event handler inner exceptions set the tasks completion source to status faulted with exception everything works fine. But when I start the service and after a while set the timeout exception with StopService API call the exception gets caught in StartService try...catch but then the API automatically triggers another call to StartService method.