To fix the issue, you need make sure that PowerPoint's native slide number and/or footer are applied to the affected slides.
To do so:
- Delete the shapes that are meant to act as the slide number and footer shapes on the affected slides
- Select each slide
- Click the Header & Footer button in the Insert tab
- Check the boxes next to Slide number and/or Footer and click Apply to apply the changes
If the slide number and/or footer shapes don’t appear, make sure they are in the layout of the slides.
If they are not, change the layout of the slides and re-insert the “Slide number” and/or “Footer” shapes.