First page on it's own spread not working in RTL mode

[yankl]

Description

This can be reproduced as follows: on the "bookreader simple demo" compare the demo as is, with what happens when you add the following line to the options object in BookReaderJSSimple.js :
pageProgression: 'rl',
(switching in each case to two-page mode)
Then, instead of the first page being on it's own spread, it's combined with the next page.

Evidence / Screenshot (if possible)
LTR mode:

RTL mode:

Expectation

Since each value in the data array is supposed to represent a "spread", and this first item contains only the object for the first page in both cases, I would except the first page to be on it's own spread whatever the setting of pageProgression.

Context

Windows 10, Chrome 120.0.6099.130, BookReader 5.0.0-77