Of course way too early for any thoughts about the cause, but it reminds me about the incident in November 2014 with an LH A321, which surprisely descended because of faulty sensors.
ASN Aircraft incident 05-NOV-2014 Airbus A321-231 D-AIDP
"During climb, 12 NM NW of Pamplona, at approximately Flight Level 310, the aircraft unexpectedly decreased the pitch autonomous and started to descend. The aircraft reached a rate of descent of up to 4,000 ft/min. The crew was able to stop the descent at Flight Level 270."