I updated the Router (DIR-890L) to FW 1.20B01 last night.
So far I am not seeing any issues with client iPhone connected to WiFi through the Expander (DAP-1860), which has been at FW 1.04 for sometime, since I wanted to correct for the KRACK exploit.
When I first reported the issue, the Expander was at FW 1.01.
I will monitor over the next few days and if I don't see any more issues, we can consider this closed out.