Low-ℓ likelihood stability as a function of sky fraction. All results are evaluated adopting the same series of LFI processing masks as defined by Planck Collaboration V (2020). From top to bottom, the three panels show (1) posterior τ estimate; (2) posterior r estimate, expressed in terms of a detection level with respect to a signal with vanishing B-modes in units of σ; and (3) χ² PTE evaluated for the best-fit power spectrum in each case.