[Flow] Error while persisting releated object with "__identity"

[Nicola Cerchiari]

Hi to all,
I'm developing my very first Flow app and let me say developers did a great
job, everything went smooth and easy.
Unfortunately today I met a problem that I can't figure out.

I have 3 models, let's say A, B  and C. A is related with B by a ManyToMany
and B is, in an identical way, ManyToMany related with C.

I managed flawlessly to have a fluid form that persists model B relating it
to existent model/s A (I needed a group of checkbox).

Then I moved to C  to B relation and I replicated the exact behaviour and
flow used for A with B, but it returns me the error:
Exception while property mapping for target type
"Namespace\Domain\Model\C", at property path "B-relating-property": It is
not allowed to map property "__identity".

Now I don't understand why this error persisting C related to B and not  A
related to B as, apart from naming, the code is almost identical.

I know I could work it around using a selectbox instead of checkboxes but I
would like to understand where the error lies.

Thank you all very much for any suggestion that could give.

Bests,
-- 
*Nicola Cerchiari*