It wasn’t my intention to answer my own question, but by the time the post was approved I had already found the root cause. Adding my findings here for anyone who may find it helpful.
It turns out the left-side hotbar was never intended to be available for free users. I came across this comment in the September 2025 release notes, which explains that the earlier versions I was using (up to the August 2025 build) still showed the hotbar due to an issue. That behavior has now been “fixed” in the newer releases, which is why the panel disappeared after the update.