Cannot unmask structured arrays values based on predicate

[paolomattioli1989]

There seems to be an issue when trying to unmask a value in a structured array based on some predicate.
I would have expected this operation to successfully unmask the entry, but it does not - see the example below.

I'm running Ubuntu 20.04.3 and python 3.8.13
This behaviour has been observed on numpy versions 1.23.1 and 1.22.4 (the two ones i've tried).
The expected behaviour (so no AssertionError raised by the below snippet) is instead observed in 1.21.5.

out = np.ma.array(np.empty(3, dtype=[('a_string', '<U30'), ('a_float', '<f8')]), mask=True)
assert not out.hardmask
out['a_string'] = ['Aa', 'Bb', 'Cc']
pred = out['a_string'] == 'Aa'
assert pred.sum() == 1
out['a_float'][pred] = 1.23

assert not out['a_float'][pred].mask.item()  # <-- fails