Hank eventually told his daughter what really happened to Janet. She went subatomic to deactivate a bomb, but was sent tumbling into the Quantum Realm.
Quantum Tunnel: With Hank Pym's help, van Dyne escaped the Quantum Realm using the Quantum Tunnel Pym had designed. She later used it to send Scott Lang into the Quantum Realm, but was unable to bring him back due to her having disappeared in the Snap.
The wife of Hank Pym and mother of Hope, Janet Van Dyne operated under the codename Wasp in S.H.I.E.L.D. but became lost to the Quantum Realm during a dangerous mission.
Though it still hasn't been explained fully why the discrepancy occurred, it could relate to the points at which the Quantum Realm is entered and exited. Perhaps, Scott Lang went deeper into the Quantum Realm, or maybe he found himself in another dimension where time moves considerably slower.
Using the powers she had obtained through her long stay in the Quantum Realm, van Dyne transferred a significant amount of quantum energy into Ghost, thus temporarily stabilizing her condition and ending Ghost's campaign against Ant-Man and his allies.
After a mission with Hank Pym, Janet disappeared into the Quantum Realm. Under the impression that she mysteriously died, Hope did not know how to deal with her passing, and her father's relative absence didn't help. In adulthood, she sought after and mended her relationship with her father.
After Janet Van Dyne is rescued from the Quantum Realm, she can sense Ava's unstable state and is able to partially heal Ava with some of her own quantum energy.
Ant-Man and the Wasp's post-credits scene ended with Scott Lang being trapped in the Quantum Realm. This came as a result of Thanos' snap dusting Hope van Dyne, Hank Pym, and Janet van Dyne, making it impossible for them to pull Scott back out.
Tony stark created a mapping system so they could find their way around the quantum realm, time is extremely abstract down there so 5 years could be 1 second or 80 years depending on where you would be.
When the three are restored to life, Van Dyne joins the Avengers in a battle against an alternate version of Thanos. Afterwards, she buys back her father's company heading the Pym van Dyne Foundation and continues her relationship with Scott Lang becoming a step-mother to his daughter, Cassie Lang.
Since Scott had used Hank's complex technology, the Pym particles, Hank and Hope were held responsible for Scott's actions and became fugitives on the run. Both were angry at his betrayal, and Hope was particularly angry that Scott didn't ask her to help, stating that he wouldn't have gotten caught if she was there.
There are touches added in—like Cassie calling Hank "Grandpa"—that help to illustrate how close this family has grown in the time since we last met them.
Through Nadia, we discover that Hank has bipolar disorder, which he has been monitoring on his own to predict when and how often his mood swings may occur.
Kang the Conqueror was exiled to the MCU Quantum Realm by his own variants in the Council of Kangs due to his attempts to eradicate them and become the sole Kang in the multiverse.
Michelle Pfeiffer's Janet Van Dyne may have had a physical relationship with Kang the Conqueror while they were both trapped in the Quantum Realm, and the pair may have even had sex. Ant-Man and The Wasp: Quantumania revealed that Janet wasn't alone when she was trapped in the Quantum Realm for many years.
A simple lab accident with poisonous gasses even led him to develop schizophrenia, which in turn led him to adopt multiple personas. At different times the original Ant-Man was also Giant Man, Goliath, Yellowjacket, and even the Wasp. (Not to mention a litany of other aliases he adopted over the years.)
It's never specifically stated but our best guess is that Janet simply felt guilty about choosing her family over all of the people she left subjugated to Kang in the Quantum Realm. If she actually acknowledged it, that would've meant having to deal with it, and she just wanted to ignore it.
Not beating around to bush, Reed said that he chose to replace Cassie because he admired Newton as an actor, and wanted someone who could keep up with Paul Rudd.
During her days of entrapment in the Quantum Realm, Janet van Dyne (Michelle Pfeiffer) (Pym's wife, Hope's mother, and the original Wasp, who was lost in the Quantum Realm for 30 years) encounters an exiled traveler named Kang (Jonathan Majors).
Starr expressed to Janet that her condition hurts and always had. Janet apologized for her sickness and told Starr that she can fix it. Janet transferred a portion of quantum energy into her, making Starr feel whole again and letting her touch Janet's hand.
Evangeline Lilly portrays Hope van Dyne / Wasp in media set within the Marvel Cinematic Universe, with Madeleine McGraw portraying the character as a child in flashbacks depicted in Ant-Man and the Wasp.
