ARIMA forecast xreg specials not finding data

[mitchelloharawild]

It looks like they exist in the env, but cannot be found by model.frame(). Ref: https://stackoverflow.com/questions/68775156/fablearima-cant-find-xreg-when-programmatically-generating-formulae

[wkdavis]

@mitchelloharawild Thanks for opening this! I am experiencing something similar when using xregs with TSLM.