The second season of "Beecham House" hasn't "vanished" so much as it never existed — and was never meant to.
From the beginning, before it ever aired, the show was being billed by its network (U.K. broadcaster ITV) as a six-part miniseries; and that's what they delivered.
A few days after the finale ended in July 2019, the show's official Twitter account even posted a pretty final statement: "We hope you've enjoyed the series as much as we have enjoyed making it."
However, there's always a way to make more if people want it enough, and in this case, one of the people who wanted it was actually the creator.
Writer-producer Gurinder Chadha was reportedly (according to Deadline.com) disappointed that ITV wouldn't give it a second season and briefly shopped it around to other broadcasters. That could explain why, as you say, it had a cliffhanger ending, despite being billed as a miniseries.
This was in 2019, though, so it appears she didn't have any luck finding "Beecham House" a new home.
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