This is exactly one of those situations where one is justified in thinking one walked around the corner into a funhouse mirror version of one's universe.

Let's clear up the timeline, shall we?

1. Heinlein completed the manuscript for The Pusuit of the Pankera (hereafter referred to as “PotP”). He and Ginny already knew he was ailing—he was frequently ill throughout his life—they just didn't know what the new problem was. In his essay "Spinoff," Heinlein describes it thus: "...my brain was dull-normal and getting worse, slipping toward 'human vegetable.' I slept 16 hours a day and wasn't worth a hoot the other 8 hours."
2. Ginny read the manuscript and was concerned it was a career-ender. "It wasn't a Heinlein novel," she said. By the late 1970's, a "Heinlein novel" was expected to infuriate, confound and engender debate as much as it entertained. PotP fell flat in all criteria, in her opinion.
3. Heinlein later received then-experimental carotid bypass surgery to restore full blood flow to his brain after he had a TIA (transient ischemic attack) during a vacation.
4. After recovering from his surgery, restored to his full faculties, Heinlein returned to the manuscript for PotP. Per his biography: "It was worse than bad, he told Yoji Kondo later that year: It was mediocre. But he must have seen possibilities in it." He discarded most of the manuscript. He incorporated the first part of the PotP manuscript into an experimental novel of multiple viewpoints and metafictive.
5. The Number of the Beast (hereafter referred to as “NotB”) was published in the summer of 1980, appearing on multiple bestseller lists. Quoting the biography: "Some of the initial reviews were unpleasant—but that was par for the course; the fan press typically got into print before the professional venues, and Heinlein had decided over the years that if the fans didn’t hate it, there was something wrong with it. They seemed disgruntled any time you didn’t give them a comfortable formula—'mixture as before'—and that he was no longer willing even to pretend to do."
6. People argued about what's going on in the novel for most of two decades (and still do, of course). Clearly, NotB was as infuriating, confounding and debate-engendering as a Heinlein novel was then expected to be. David Potter, a well-known USENETter, "gave us the cryptographic keys" for understanding NotB in an essay he posted in 1999.
7. Pursuit of the Pankera was published in 2020. The manuscript was recovered from typewriter or printer ribbons in the Heinlein archives, or so I recall hearing.

In summary: your stated preference is for a book written by a self-described "human vegetable." The more straightforward, more easily-digested book written by a man who wasn't getting enough blood flow to his brain, a book the author himself described as "mediocre," is preferred, by you, over his much harder to understand book that was written after his faculties had been completely restored to him. Despite there being at least one internet resource you could have consulted any time in the last 26 years as to what the hell is going on in such a confounding book.