I think thunked fields are breaking my previously working code

[edk]

Dear Guixers

This is a follow-up on

https://lists.gnu.org/archive/html/help-guix/2022-01/msg00000.html

Using the REPL, I traced the bug on my code trying to update fields of
an operating-system records, which on gnu/system.scm are declared as
"thunked".

Guile is not my primary language. I would really appreciate some
pointers here:
- Why is it that I can't update those fields ?
- I was able to ~1 month ago, but can't do that now. Why ? Is my
use-case unsupported ? Should I expect other breaking change in the
future ? i.e. was I using a non-public interface ?
- How can I update those fields ?

For example, given that "minimal-container" is an operating system, I
can do the following:

(set-fields minimal-container ((operating-system-host-name) "toto"))

But not:

(set-fields minimal-container ((operating-system-label) "toto"))

while both fields are supposed to be strings, but label is "thunked".

Thanks in advance for any help.

Cheers,

Edouard.