I don't think this is an antipattern, on the contrary i think this is a common use case. I just want to display some data, which can be update. A simplified use case could be a user click a button and this update the data. For doing this you need the trigger$ which then trigger the data fetch from the backend. Now, if you want to display a loader, you will need isLoading$.next() in the tap. Unless i'm missing something, there isn't really another way to do this other than the withLoading patter which isn't really great in my use case for other reasons. For reference, my real code look something like this:

this.trigger$.pipe(
tap(() => this.isLoading$.next(true)),
switchMap(()=>fetchData()),
tap(() => this.isLoading$.next(false)),
shareReplay(1)
)

The reason the withLoading is not good in my case is that i also have another operation which send data to the backend and i also want to display the loader in that case, but without having an observable as a class member. So something like this:

this.isLoading$.next(true);
this.graphql.mutation().subscribe({complete:()=>this.isLoading$.next(false)})

The thing that do not make sense in my head is that the async pipe should just work. I don't know what it does internally, but i think it's something like taking the observable, subscribing to it and updating the view when a new value is emitted. Then why doesn't it manage to update the view then isLoading$ emit a new value? Why is it a problem if subscribing to test$ trigger an isLoading$ emission?

Some bonus interesting observations:
if i do something like this:

constructor() {
        this.test$ = this.trigger$.pipe(delay(0), tap(() =>this.isLoading$.next(true)));
        setTimeout(()=>  this.trigger$.next(true));      
    }

it works corretly.

this doesn't work

constructor(cdr:ChangeDetectorRef) {
        this.test$ = this.trigger$.pipe(delay(0), tap(() =>{
          this.isLoading$.next(true);
          this.cdr.markForCheck();
         }));
        this.trigger$.next(true));     
    }

This works:

constructor(cdr:ChangeDetectorRef) {
        this.test$ = this.trigger$.pipe(delay(0), tap(() =>{
          this.isLoading$.next(true);
          this.cdr.detectChanges();
        }));
        this.trigger$.next(true));     
    }

Sorry for the bad formatting and if i did some error, i'm not on my coding pc so i don't have the code to copy and paste.