I've updated the ticket subject for the real problem. Computed fields are initially restored as regular ones (with missing values) and then altered to become computed. Index creation misses the new definition and deals with the older one, causing NULL keys to be stored in the index.
v2.5.2 is also affected, you just don't see it because the engine does not use the index: plan SORT is used instead of ORDER. It was a regression but it was fixed (in v2.5.4, I think). Now the restore issue becomes visible.