Doctor Who: Flux perfectly demonstrates how The Doctor's habit of leaving companions is ingrained into the character's history - even the bits she can't remember. Everyone knows the hazards of becoming The Doctor's companion. You will be shot at by grotesque aggressors. You shall get frequently patronized by an alien with questionable fashion sense. And most importantly of all, there's a strong chance of being dumped and forgotten when the partnership comes to an end. Some companions are fortunate enough to depart the TARDIS on their own terms, but many more are forced to live a mundane life on Earth while The Doctor continues gallivanting with a replacement at his side.
By far the most emotive explanation of The Doctor's long-standing abandonment insult comes from Sarah Jane Smith in Doctor Who season 2's "School Reunion." She confronted The Doctor, pleading, "You know what the most difficult thing was? You took me to the furthest reaches of the galaxy. You showed me supernovas, intergalactic battles, and then you just dropped me back on Earth." Many other TARDIS guests could be forgiven for feeling the same way - Susan, Jo Grant (who voiced her unhappiness in The Sarah Jane Adventures), Peri, Ace... Adric would probably complain too if he hadn't died horribly.
Doctor Who: Flux proves The Doctor has been making companions feel worthless since before she can even remember. In "The Vanquishers," Jodie Whittaker's Thirteenth Doctor presses Karvanista about their time together in Division, before suddenly realizing the Lupari was, once upon a time, her TARDIS companion. Echoing Sarah Jane and Jo Grant's sentiments, Karvanista complains, "There was a time I'd do anything for you, but you left me... Then you come crashing back in as if nothing had ever happened."
Doctor Who has already established how many ditched companions feel aggrieved that The Doctor never swung by for a visit, but tracing this problem back to the Division years - before The Doctor even became the renegade Time Lord our universe knows and loves - is a damning indictment of how many companions have been ghosted by Doctor Who's flighty protagonist. The list was long enough before, but with potentially thousands of forgotten Timeless Child regenerations hiding in The Doctor's fob watch, how many like Karvanista are out there, longing to one day hear the TARDIS' vworp one more time?
One important caveat is The Doctor's erased memories. As seen in "The Timeless Children," The Doctor was subjected to memory wipes while working for Division, and doesn't remember anything before her William Hartnell-shaped regeneration. It's possible, therefore, that The Doctor would've reunited with her fluffiest (presumably, who knows?) companion if Division hadn't made her forget all about the Lupari pooch.
But abandoning companions involuntarily doesn't necessarily justify the hurt. Jamie McCrimmon, Zoe Heriot and Donna Noble all got purged of their TARDIS memories before The Doctor was forced to leave them. A cruel fate indeed - but all three companions were being punished, directly or otherwise, for The Doctor's own actions. Division may shoulder much of the blame but, as far as can be gleaned from flashbacks, The Doctor was recruited willingly. And would the Lupari have been reunited with its master if The Doctor's memories weren't wiped? Given the track record of her past 14 regenerations, probably not. Could Yaz be next on Doctor Who's list?