  SMC X-1 is a high-mass X-ray binary showing superorbital modulation with an
unstable period. Previous monitoring shows three excursion events in
1996--1998, 2005--2007, and 2014--2016. The superorbital period drifts from >60
days to <40 days and then evolves back during an excursion. Here we report a
new excursion event of SMC X-1 in 2020--2021, indicating that the superorbital
modulation has an unpredictable, chaotic nature. We trace the spin-period
evolution and find that the spin-up rate accelerated one year before the onset
of this new excursion, which suggests a possible inside-out process connecting
the spin-up acceleration and the superorbital excursion. This results in a
deviation of the spin period residual, similar to the behaviour of the first
excursion in 1996--1998. In further analysis of the pulse profile evolution, we
find that the pulsed fraction shows a long-term evolution and may be connected
to the superorbital excursion. These discoveries deepen the mystery of SMC X-1
because they cannot be solely interpreted by the warped disc model. Upcoming
pointed observations and theoretical studies may improve our understanding of
the detailed accretion mechanisms taking place.
