BUG #16755: A specification or a bug? Digit drop on CAST from DOUBLE PRECISION to NUMERIC.

The following bug has been logged on the website:

Bug reference: 16755
Logged by: Kobayashi Hisanori
Email address: hisanori(dot)kobayashi(dot)bp(at)nttdata(dot)com
PostgreSQL version: 12.4
Operating system: Windows 10
Description:

Is this a specification? Or is it a bug?
In PostgreSQL 12.4 for Windows.

It would be understandable if the CAST from NUMERIC to DOUBLE PRECISION
would digit drop, but I don't know if it's a specification or a bug to digit
drop on CAST from DOUBLE PRECISION to NUMERIC. (translated by DeepL)

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postgres=> select (123456789012345::double precision * 11) ;
?column?
------------------
1.358024679135795e+15 <-- *** No Problem(No digit drop)

postgres=> select (123456789012345::double precision * 11)::numeric ;
numeric
------------------
1358024679135800 <-- *** Digit drop
-----