Query question

I seem to have a problem with a specific query:

The inside query seems to work on it's own:

select prefix
from john_prefix
where strpos(jpt_test.number,john_prefix.prefix) = '1'
order by char_length(john_prefix.prefix) desc limit 1

but when I execute it with this:

UPDATE
jpt_test
set
number = substring(number from length(john_prefix.prefix)+1)
from
john_prefix
where
prefix in (
select prefix
from john_prefix
where strpos(jpt_test.number,john_prefix.prefix) = '1'
order by char_length(john_prefix.prefix) desc limit 1
) ;

table contents are as follows

john_prefix table:

prefix
---------
123
234

jpt_test table:

number
-----------
1237999999
0234999999 <<< supposed to have no match
2349999999

Am I missing something here? Any help will be appreciated.

Regards,
JPT