HERE WeGo navigator public transport is not shown anymore

Not sure what you are talking about. The HERE WeGo app has vector graphics and no tiles.

I referred to the layers/tiles icon in the lower right corner in HERE.

And no, they don't use vector graphics but bitmap tiles: I observed that (with only some map content cached), public transport can be visible from a high level perspective, and, as you zoom in, the public transport stuff can disappear (as the content is not cached - and as there is no internet connection, it can't download the missing parts). After zooming in a bit more, public transport is visible again (present in the cache again).

If they would have used vector data, they could display the colorful public transport lines on all zoom levels, just rescaling the more detailed data for a high-level perspective.

The labels (line numbers, ...) seem to be independent and vector-graphics-like from the map background, though.

Tiles for public transport map are working..However, itineraries search aren't returning any results anymore in the Public Transport section.

I had problems fetching layers on Google Map with another different app (a carsharing app, which uses Google Maps to display this layer).

restarting the android layer fixed this.