I’m not sure, either. I’ll have to go back and “unfix” it so that I can test.
I guess my general feeling is that data imputation should be the “marked” (non-default) mode of operation, instead of the unmarked (default).
If you expect there to be no missing data, then your model fails in very hard-to-track ways when imputation happens (or doesn’t happen, which seems to be my case) instead of getting a warning about missing data.
I don’t think that I’m wrong in this – I suspect most users don’t expect imputation, but I could be wrong.