lightning jack adapter problem with headphone controls

I have had an iPhone 8 for about a year, and I use the supplied lightning to jack adapter.

Recently, I noticed that the headphones control had stopped working. I thought it might be something with the headphones until I did more testing:

1) on an iPhone 6 using the jack, they work so it is not a problem with the headphones

2) on the same iPhone 6 and my iPhone 8 using the adaptor, they don't work

3) with standard Apple headphone, the behaviour is similar (except that when I plug the adapter it launches Siri)

Both iPhone run iOS 12.1. I suspect that the problem came with that update, but I am not really sure. Is anybody experiencing the same problem?