I think it's very possible that Yhwach saw Ichigo activate his Bankai and then disappear.
Given Ichigo's relationship with fate and strength and authority, it'd be very fitting if Tensa Zangetsu's ability is "no one will tell me what I can or will do" and thus he's invisible to The Almighty. An ability of limitless possibilities.
Gotta nip that one in the bud indeed















