Regardless of issue I highly recommend reading try following all the best practice tips here to rule those issues so not waste developer’s time troublesooting → Zigbee networks: how to guide for avoiding interference and optimize for getting better range + coverage
It might as well good idea to also try changing Zigbee channel as well as verify that you are using static WiFi channel(s) that do not overlap → https://support.metageek.com/hc/en-us/articles/203845040-ZigBee-and-WiFi-Coexistence
Also, if you have already done so already suggest upgrading to the latest available deconz firmware to the ConBee adapter → https://github.com/dresden-elektronik/deconz-rest-plugin/wiki/Update-deCONZ-manually