PIO Core: build offline (w/o Internet connection)

PlatformIO Core
3

I’ve been investigating further. Take this project:

[env:d1_mini]
platform = https://github.com/platformio/platform-espressif8266.git#v1.5.0
board = d1_mini
framework = arduino
build_flags = -g -DMQTT_MAX_PACKET_SIZE=400 ${env.ESPURNA_FLAGS}
lib_ignore =
lib_deps =
    Brzo I2C
    https://github.com/krosk93/espsoftwareserial#a770677
    https://github.com/marvinroger/async-mqtt-client#v0.8.1
extra_scripts = extra_scripts.py

I build the project with internet connection, then I disconnect from the Internet and build it again. It fails with “Error: You are not connected to the Internet. If you build a project first time, we need Internet connection to install all dependencies and toolchain.”.
But then if I remove the async-mqtt-client dependency it works!

Trying with a bigger project (ESPurna) with several dependencies in the lib_deps it works if I comment out the async-mqtt-client and the fauxmoESP libraries:

#    https://github.com/marvinroger/async-mqtt-client#v0.8.1
#    https://bitbucket.org/xoseperez/nofuss.git#0.2.5

If I do so it compiles successfully since those libraries are actually on .piolibdeps. If I enable them it fails…