Infinite Scroll: Jetpack Portfolio and Testimonial (CPT) Paging Issue when Infinite Scroll is Enabled

#### Steps to reproduce the issue
1. Go to `Jetpack > Settings > Writing` and enable **Custom Content Types** module.
2. Please add some Jetpack Portfolio and Testimonial Posts
3. Got to `Jetpack > Settings > Appearance` and enable Infinite Scroll module.
4. Go to `Settings > Reading` and set **10** for Blog pages show at most.
5. Go to `Settings > Writing` and set **1** for Portfolio and Testimonial pages display.
#### What I expected for Portfolio CPT
- Portfolio archive pages e.g. (http://example.com/portfolio/) should owner the `Settings > Writing` value.
- There should be **1** portfolio post on each load of Infinite scroll.
#### What happened instead for Portfolio CPT

It is loading **10** portfolio posts instead of **1**
#### What I expected for Testimonial CPT
- Testimonial archive pages e.g. (http://example.com/testimonial/) should owner the `Settings > Writing` value.
- There should be **1** testimonial post on each load of Infinite scroll.
#### What happened instead for Testimonial CPT
- It is loading **1** testimonial post on first load which is as expected.
- But, It is loading **10** testimonial posts instead of **1** on second load.

_Please note that Jetpack Portfolio and Testimonial paging is working as expected when Infinite Scroll is disabled._

Thanks

Reproduce the issue through Jetpack Custom Content Types, Infinite Scroll, and the Writing, Appearance, and Reading settings using Portfolio and Testimonial archives. Trace how the archive page-size setting is applied on the initial and subsequent infinite-scroll loads; done means both post types load the configured number on every load, with paging still correct when Infinite Scroll is disabled.

Written by the indexing model from the issue text.