ISBAT Carousel shows up as grey image

### Describe the bug:
The previous and next buttons are invisible, only the center image shows up and it is greyed out.

### To reproduce:

### Expected behavior:
See carousel with the center image at full opacity

### Screenshots:
http://aboutrc.com/error.png

Issue replicated in local environment as well

### Isolating the problem:

- [x] This bug happens with no other plugins activated
- [x] This bug happens with a default WordPress theme active
- [x] This bug happens **without** the Gutenberg plugin active
- [x] I can reproduce this bug consistently using the steps above

### WordPress version:
5.9.1

### Gutenberg version:
/

Initially reported on WordPress.org: https://wordpress.org/support/topic/carousel-shows-up-as-grey-image-no-other-images-show-up/