Hi! We have a really similar issue. We wanted to change from legacy BLE advertisement to the BLE 5 advertising extension. We tried advertising with different development kits (nRF and ST chip) and devices and we see the same behaviour with all of them. We can even reproduce it if we advertise from an Android device with the nRF connect app. So we are trying to connect with iPhones to a BLE peripheral advertising with the advertising extension (CONNECTABLE, secondary PHY: 2Mbps). From the iPhones (BLE central) side it looks like it takes really long to connect, we don't get any errors. From the peripheral side it looks like the iPhone is producing a connection establishment failed error, and tries to connect again immediately. And this happens multiple times in a short period of time, and finally after many seconds this reconnecting loop breaks and the connection is established successfully. We tried it with 10 different iPhone models that supports the advertisement extension from iPhone XS to iPhone 15 pro. The newer models (from iPhone 13) can connect much faster, with less reconnection attempts. The worst performing is the iPhone XS. We tried to investigate this issue with sniffers, measuring connection times and research. We never saw the issue when we were trying to connect to the same peripheral with an Android phone or other BLE devices. That's why we believe it's not a problem on the peripheral side. We also found that Apple devices violate timing values when they are trying to connect to a peripheral using the advertisement extension. Reference: https://devzone.nordicsemi.com/f/nordic-q-a/94182/issue-connecting-from-ipad-air-to-extended-advertising-set-of-nrf53/ In our test we saw that the performance changes in a significant amount when we change the advertisement parameters, like the interval and the secondary skip max value. But the connection - failed to establish connection loop is still present, but broken faster with specific advertisement parameters.