Sorry, this is our bug and has been fixed in Fixed an issue when Library Dependency Finder (LDF) ignores custom "l… · platformio/platformio-core@5cc9a32 · GitHub
We plan to publish bug fix release of PIO Core 4.2.1 next week, A temporary solution is to use lib_ignore = AsynTCP or lib_compat_mode = strict in platformio.ini.
[env:esp12e]
platform = espressif8266
board = esp12e
framework = arduino
monitor_speed = 115200
lib_deps = MFRC522, ESP Async WebServer, ESP8266TrueRandom
lib_compat_mode = strict
You can also switch to PIO Core development version via pio upgrade --dev command.