2.8.1: Compiler produces invalid ozn files

[informarte]

I noticed that the MiniZinc 2.8.1 compiler produces invalid ozn files for some problems from past MiniZinc challenges:

vaccine:

> minizinc -c vaccine.mzn v946.dzn
> minizinc --ozn-file vaccine.ozn
Error: type error: no function or predicate with name `\0@_toString__constrId_0_VACCINE' found, did you mean `_0@_toString__constrId_0_VACCINE'?

travelling-thief:

> minizinc -c ttp.mzn data/n10_k3_c5000_l10000_u10100_r46.dzn
> minizinc --ozn-file ttp.ozn
Error: type error: field access of a record must use a field identifier

generalized-peacable-queens:

> minizinc -c peaceable_queens.mzn n8_q3.json
> minizinc --ozn-file peaceable_queens.ozn
Error: type error: no function or predicate with name `\5@_toString__constrId_0_Armies' found, did you mean `_5@_toString__constrId_0_Armies'?

(MiniZinc 2.7.6 produces bad ozn files, too, but the error messages are different.)