Hmm good question - I’ll check a little later today. As far as I know I just have the “latest” ESPHome version installed via pip, and nothing else special about the build environment.
However, I might have included ArduinoJson-esphome at some point and it’s just left over in the build folder!
As a quick thought, maybe try adding it as another library to include under ESPAsyncUDP in the YAML? Just with “- ArduinoJson-esphome” (edit2: this should be " - ArduinoJson-esphomelib")
Edit: also I’m pretty sure it’s v5 of the ArduinoJson-esphome library, which isn’t the latest. If I’ve somehow got that lingering around, but v6 is what ESPHome pulls now and it’s not compatible, definitely need to update that!