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.kobayashi.bp@nttdata.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)

----- 
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
-----