Behemoth is very close to class 8 & you likely chose to repair to blueprint before repairing the blocks.
The damaged blocks take more size class than repaired ones for some reason, so when all the blocks were replaced, but some of them were still broken, the ship class went above 7.5
At least this is how I explained myself after it happened to me
Could be wrong, but the issue did not happen again after I started repairing the blocks first, before repairing to blueprint.