Exponentiation confusion

In trying to answer an SO question I ran across this:

Postgres version 14.5

select 10^(-1 * 18);
?column?
----------
1e-18

select 10^(-1 * 18::numeric);
?column?
--------------------
0.0000000000000000

Same for power:

select power(10, -18);
power
-------
1e-18
(1 row)

select power(10, -18::numeric);
power
--------------------
0.0000000000000000

Why is the cast throwing off the result?

--
Adrian Klaver
adrian(dot)klaver(at)aklaver(dot)com