Ah, that can be checked easily. Please run the “Verbose Build” task (or pio run -v). The dependency graph will then show the full paths.
Dependency Graph
|-- <Encoder> 1.4.1 (C:\Users\Maxi\Documents\PlatformIO\Projects\esp_test\.pio\libdeps\teensy2pp\Encoder)
|-- <Control Surface> 1.2.0-4 (C:\Users\Maxi\Documents\PlatformIO\Projects\esp_test\.pio\libdeps\teensy2pp\Control Surface)
| |-- <SPI> 1.0 (C:\Users\Maxi\.platformio\packages\framework-arduinoteensy\libraries\SPI)
| |-- <Encoder> 1.4.1 (C:\Users\Maxi\Documents\PlatformIO\Projects\esp_test\.pio\libdeps\teensy2pp\Encoder)
| |-- <FastLED> 3.3.3 (C:\Users\Maxi\.platformio\packages\framework-arduinoteensy\libraries\FastLED)
| | |-- <SPI> 1.0 (C:\Users\Maxi\.platformio\packages\framework-arduinoteensy\libraries\SPI)
| | |-- <SoftwareSerial> 1.0 (C:\Users\Maxi\.platformio\packages\framework-arduinoteensy\libraries\SoftwareSerial)
| | |-- <DmxSimple> 3.1 (C:\Users\Maxi\.platformio\packages\framework-arduinoteensy\libraries\DmxSimple)
| |-- <SoftwareSerial> 1.0 (C:\Users\Maxi\.platformio\packages\framework-arduinoteensy\libraries\SoftwareSerial)
If SoftwareSerial doesn’t point to framework-arduinoteensy\libraries\SoftwareSerial, that’s an error. What does your output say?