FetchIssuersRequest only returns issuers for Ideal (https://github.com/thephpleague/omnipay-mollie/blob/master/src/Message/Request/FetchIssuersRequest.php) but other methods like KBC/CBC Payment methods also require an issuer https://github.com/craftcms/commerce-mollie/issues/33#issue-805347295
FetchIssuersRequest only returns issuers for Ideal
(https://github.com/thephpleague/omnipay-mollie/blob/master/src/Message/Request/FetchIssuersRequest.php) but other methods like KBC/CBC Payment methods also require an issuer
craftcms/commerce-mollie#33 (comment)