This might be true of that route but BA have sold itineraries with flights operated by non-OW airlines for as long as I remember (that's a long time) including where connections on OW airlines exist. There is absolutely nothing new about it.

As to why this now pops up on your itinerary to TPE, essentially can be either a new agreement that wasn't there before, or CX reducing frequencies leaving space for others, or an issue with the CX availability system which makes the CX flight (or BA codeshare version) look unavailable to the system by mistake so it goes for the next alternative, or changes of times or increased alternative offer which make the alternatives look like better connections. Some of those things would be temporary some could be permanent, and I am sure there are also other possible explanations, but the bottom line is really "nothing new" even if it's not a specific route on which you were used to seeing this.

You can of course call BA to book routings on CX tha do not show if you want them.