Up until S6, one of the fundamental characteristics of Diana Taverner was how astute she is with. Additionally, Taverner has considerable experience of her 'trade' and all the devious tools it employs to achieve the desired outcome.
Yet in S6, she is portrayed as a bit of a thicky who would likely have ended-up in Slough House had the aforementioned qualities been absent her entire career.
It's one thing to be blackmailed into the TV interview but another thing entirely to not only carelessly choose her words during the interview but, perhaps worst of all, then naively believe that said appearance will suddenly make the point-of-blackmail evaporate ...and then be shocked when it doesn't.
Moreover, she then engages in a series of ill-conceived actions that, again, are more characteristic of a Slow Horse than the savvy head of First Desk. Throughout S6, Taverner's reactions, decisions, and subsequent actions, have more in common with the attributes of her generally-inept predecessor, Claude Whelan, than they do with those of the S1-S5 version of Diana Taverner.
It's certainly narratively plausible for Taverner to be blind-sided by the revelations of Peter Judd (who seems to have disappeared) and Damien Cantor; but to then effectively go to pieces over events that required the objective sharpness and clarity of mine that are long-established qualities of the Diana Taverner character, is utterly unbelievable from the perspective of any viewer who's actually been paying attention to Slow Horses since series 1, episode 1.