The intention of the GDPR is to minimize the amount of personally identifiable information (PII) you store. So the GDPR never tells you that you have to log certain events. The simplest way to make sure you comply with the GDPR is to simply not store any PII at all. But that might of course conflict with legitimate business interests and with other legal obligations you might have. So when you do store PII, then the GDPR regulates under which conditions you are allowed to store PII, for how long and what you are allowed to do with that PII.
So when you receive a GDPR request from a customer to see their data, then you can say that you only have that one timestamp of their registration, because you didn't log their subsequent logins (assuming this is the truth).
I hope your privacy policy says that you store all that PII on registration and that you have some good arguments why storing that information is a "legitimate interest" of your organization.