RB2011UiAS + mAP Lite (PoE) as AP - loosing internet on WiFi

Hi there.
I have a RB2011UiAS for quite some time now. Generally works great despite pretty old FW: 3.22…
Anyway lately there was a need to add an AP to the network. So why not a compact one using router’s PoE output. I’ve decided to try mAP Lite.

I had to twist some IP networking since I want some kind of separation of WiFi vs rest of LAN.

RB2011 LAN IP = 192.168.0.111
RB2011 Eth10 (PoE port) = 10.10.20.111
mAP Lite Eth1 = 10.10.20.20
On wlan1 I’ve made another subnet = 10.10.200.1 (dhcp pool: 10.10.200.2-10.10.200.20)
Now I get reports that sometimes there’s no internet on WiFi. For now I’m unable to troubleshoot it on WiFi client side, but here’s what I’ve noticed.

I did some port forwarding+firewall rules so I can access both RB2011 + mAP Lite via Winbox remotely. I have no issues having both winbox sessions launched simultaneously (no connection errors etc. - so I assume that internet connection to/from mAP itself is stable enough). One thing makes me wondering: if I’ll launch ping packets to lets say 1.1.1.1 from RB2011 - works perfect - no losses. If I run ping from mAP - works fine aswell. HOWEVER if I’ll launch ping on both devices simultaneously - all ping packets = timeout on mAP device. Situation’s the same even if pinging DIFFERENT addresses from RB2011 and mAP. Is that uhm… normal by any means that I can’t run ping packets from both MT’s? Perhapse that might be a clue for some WiFi issues aswell…