Thank you both for the inputs. I was actually looking for a way to debug what's happening on the WiFi and why the Android client gets disconnected.
The optimized build for the DIR-86L, while I do appreciate the development efforts, is something I wouldn't use on daily basis because I really need stability.
Meanwhile I managed to find out what's causing the issue and it's not related to OpenWRT.
The Android 9 based LineageOS apparently has issues when both BlueTooth and WiFi 2.4 are used at the same time and it's definitely a firmware/driver problem for the particular client (Samsung Phone). On other Android clients, including LineageOS and Stock ROMs I don't have this issue.
Sorry for the noise. I'll put this thread on resolved.