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