Thanks for letting us know about this, and I'm sorry for the delayed response. Could you let me know which model your device is (e.g. Z01KD, Z01KDA or Z01KS) so that we can take a closer look at this issue?
I can confirm that Geekbench 5 is using the correct core and frequency (approximately 2.8 GHz) when running benchmarks on the Xiaomi Poco F1. The "Base Frequency" that is displayed on the result is provided by the device, and for Android devices with multiple core clusters it only displays the frequency for one of the clusters. I apologize for any confusion caused by this, and for my delayed response. I will pass this along to my team to see if we can clarify the Base Frequency metric in the future.