See here: https://en.wikipedia.org/wiki/Betelgeuse#Variability

And from the original NOIRLab link: “This discovery answers the longstanding mystery of the star’s varying brightness”.

The system’s brightness decreases when the companion star swings around behind Betelgeuse. It also dips when Betelgeuse goes behind the companion star but much less so because Betelgeuse is so much larger.