Prediction interval - Linear regrission

I have some guesses, but it's hard to troubleshoot without your model and an example dataset. Could you include a dataset and your model code so your question is reproducible?

If you've never made a reproducible example (i.e., "reprex") before, you can see an explanation and some suggested reading material here:

Note that one reason for getting predictions for the original dataset instead of the new dataset for lm objects is if you don't have a variable in the new dataset with the same name as the explanatory variable in your model.