It's not uncommon for banks and fintech companies to offer incentives during promotional campaigns, to get new customers, deposit even if it means they might incur a loss. However, it's essential to note that such offers are typically short-term, in this case it is only lasting for one month.
Additionally, these promotions is likely to have a stipulation that the saving must be maintained for a longer period one year (say) preventing withdrawal within the first month.
In your situation, since you plan to keep your money in the bank regardless, the IDR1.2 million bonus is essentially an extra benefit. If the annual equivalent rate (AER) offered is competitive in the market, it becomes a favourable proposition, providing both the bonus and a leading interest rate.
Sea Bank is owned by a reputable company Sea ltd. While Sea Bank claims to be covered by the Indonesian government deposit guarantee scheme LPS (Lembaga Penjamin Simpanan) on their website,
I couldn't verify this information on the LPS website (
https://www.lps.go.id/).
It's crucial to be aware that in Indonesia, if the bank subscribe to LPS,
theoretically, any savings account up to IDR2 billion is guaranteed by the LPS, the Indonesian government. However, keep in mind this is Indonesia.
The way I see it like this. If you were to deposit IDR120m in another Indonesian bank, you would likely encounter similar risks. But here you get bonus of IDR1.2m. BUt this is not a suggestion, just personal opion. Do it with your own risk.