I’m having the same issue.
My Fairphone 2 which is running LineageOS 15 (Android 8.1) does not have this issue - can do AAC and LDAC.
I’m unclear as to what to do next.
Doing more research. This post about aptX on LineageOS seems related and seems to confirm what you’re saying, @JeroenH
I’m not happy with this outcome though. I’ve sent a support request.
Edit: Wikipedia says LDAC is open source and included in AOSP. So what you said about aptX doesn’t apply to LDAC.