I suggest it may be worth looking at the previous flight FRA to LHR, P35130 vertical profile.
After six minutes of flight it reached just over 12,000 ft and then descended to 10,000 ft for fourteen minutes before climbing to a cruise level (FL240).
During the LHR to ATL, divert to FRA ( same flight number), it never got higher than 9000 / FL090.
Fairly obvious problem I would surmise.