User with PaidSubscr. ID behavior differently

I had a website that allow user to upgrade their membership.
but the button behave differently when user’s Paid Subscr. ID is defined / empty.

Here is the scenario,
User A purchased level 1 with $300 1 year subscription.
in the mid of the year, he decided to update his membership to level 2, which is caused $400 per year.
in this case, A will be charged a 1 time fee of $250 for the first year, and recurring fee $400 for the following years.
the one time fee calculation is basically just the first year level 2 fee – the number of days used for level 1 fee.
so basically $400- $150 ( $300 / 2 )

Everything works fine, when i test on the user that i created manually.
i able to see the terms in paypal showing.
$250 for the first year
Then $400 for each year

however, when user’s Paid Subscr. ID exist.
in paypal,
the terms only show
$400 for each year.
where the 1 time payment is gone.

both of them having the same s2member paypal button shortcode.
is that a bug or special setting can bring back the 1 time payment ?