Join Requests in the EU868 region must come via the 3 default channels (868.1MHz, 868.3MHz or 868.5Mhz). The default channels support only SF7 to SF12 LoRa modulations, at 125KHz.
Since your join request arrives via an extra channel (868.8MHz), it is dropped by the Network Server, as your end device is not allowed to use that particular frequency for joins.