Only single linebreak is taken into account when having multiple linebreaks in RTE

Raising this on behalf of CSP-59741 (restricted to public):

When i'm creating a new page, i'm using 'MAJ + Enter' to create a <br>.
On Rich text view, i can see this BR correctly but when i'm looking on 'Preview' view, Line Return (Shift/Enter) are not correctly displayed.

On my example (see attachment 1 - EnterShiftPage - Confluence.jpg), i have made 0/1/2/3/4 shift enter between all text entered.

As you can see in 2 - EnterShiftPage - Confluence - FireBug.jpg, a <br> is created after entering Shift/Enter.
On 3 - EnterShiftPage - Confluence Preview.jpg, you can see that, when i set 2 or 3 or 4 Shift/Enter, only one <br> is taking into account.

After saving the page, see 4 - pAgeGCA - Confluence Save.jpg, the <br> between "0 Shift Enter" and "1 Shift Enter" is not correctly displayed, as i said previously when i set 2 or 3 or 4 Shift/Enter, only one <br> is taking into account.

I can reproduce this in 3.5 but not in 4.0.