When there is more than one payment condition configured for a payment system, and one payment condition other than the one with the 1x installment option is configured as the default, the 1x installment option does not appear on the checkout page. In such cases, only the 1x installment option is removed from checkout.

To simulate this issue, follow these steps:

1. Go to the admin panel and configure three payment conditions for a credit card payment system, for example, Visa. Payment condition A could have installments of 1x-3x, condition B with installments of 4x-8x, and condition C with installments of 9x-12x.

2. Next, go to the checkout page and notice that all installment options appear on the checkout page.

3. Then, set payment condition B or C as the default and go to the checkout page again. You'll notice that the 1x installment option disappears from the checkout page.

4. However, if you set payment condition A as the default again, the 1x installment option will reappear on the checkout page.

To resolve this issue, you can set the lowest installment payment rule option to default or remove the default configuration by deleting the payment condition with this configuration and configuring a new payment condition without the default flag.