"#+STARTUP: hideblocks" won't work. Can't find offending package/setting

Question

After I upgraded to Org Mode 9.4.5, I learned about the startup option hideblocks, but it won't work as advertised for me.

OK, when I start emacs with the flag -Q, it did, so I went hunting for culprits in my .emacs. All suspicious-looking settings and packages have been removed, but hideblocks still doesn't work.

Oddly enough, when I visit a test file with #+STARTUP: hideblocks, the variable org-hide-block-startup do indeed change from nil, which is default, to t. Despite this, the block remains unfolded!

I can make all blocks fold on start-up, if I set org-hide-block-startup with a local variables section at the end of the file, but I consider that to be an ugly, last resort solution.

Is there a smart way of tracking down when and where the hideblock setting is reversed, intercepted, toggled, or whatever happens?

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Addendum April 4:

Since writing the above, I stumbled upon one additional interesting symptom. When I was led astray with the theory that my Org Mode installation was somehow faulty, I took a look at the installation instructions in the info documentation. Among other things, it says the following about an org mode installation with elpa:

Important: You need to do this in a session where no ‘.org’ file has been visited, i.e., where no Org built-in function have been loaded. Otherwise autoload Org functions will mess up the installation.

So I did M-x list-packages, threw out org mode, restarted Emacs, installed org mode anew, and visited my test file.

Lo and behold, my block was folded at start-up.

However, the effect was short-lasting, and trying to open the same file later did not work as expected.

Answer 1

The solution is much simpler than suggested by my own theories (see above). It turns out that the intended effect of #+STARTUP: hideblocks is overruled by the initial visibility setting #+STARTUP: showeverything, which is now default.

So, by combining #+STARTUP: hideblocks with #+STARTUP: showall or (equivalently) #+STARTUP: nohide, the desired effect can be obtained.

N.B. "nohide" is hiding pretty well in the info documentation, but can be found if one looks at the documentation for the variable org-startup-folded.