Had the same problem, resolved via Internet Banking Secured Mail within 3 days.
So, apparently those that have completed 3 rides and deleted their numbers from Transitlink ABT website won't receive rebate automatically.
POSB uses active/registered numbers in ABT web to query 3 first rides, which I believe is incorrect method.
However I managed to get back my rebate by providing them info on which date the bank deducted my debit card (with txn code and description), which they cross checked with Transitlink data)
Lesson learned:
1) Don't remove your card info from ABT website before rebate in.
2) POSB rebate is really hard to earn.