We upgraded our SSL certificate for api.amplitude.com, as this certificate had expired. The new bundle that our provider issued contained a different validation certificate.
This caused problems for an extremely small number of iOS/Android SDK customers who pin the root certificate (an uncommon practice). These customers' applications did not recognized our new certificate, and were unable to send events to api.amplitude.com. Fortunately, we were able to have our provider issue a new certificate that used the old validation certificate. We installed this on our server on 2/18, which should have resolved the issue.
In most cases, the affected SDKs should have sent any events once the correct certificate was uploaded. Our SDKs cache up to 1,000 customer events and will retry until they are successful. However, in a few cases, events may have been lost. We encourage any customers to reach out to us if they suspect they may have been affected.