section-5 (https://drafts.csswg.org/css-text-3/#line-breaking) talks about soft and forced line break but says nothing about the effect, i.e. the rendering. Is it the same in both cases? That is, if a line is wrapped at some place, should the resulting lines appear as if it was a forced break at the same place?

As for bidi texts, if a line break occurs inside a phrase that is progressing in opposite direction, wrt the surrounding context, the break (rightfully) does not end the "embedded" base direction, unlike HTML <br>.
As a consequence, if the next line begins with a directionally ambiguous character, the rest of the phrase takes the global opposite base direction.
Example (in RTL context):

The 1st two lines result from a text wrapped (soft-break) before the number 2. The whole Latin string flows LTR. Then the same text with forced-break at the same position. In the last line, the rest of the Latin string flows RTL.

Looks odd to me.

Source:

<div  dir="rtl" style="width: 50%;">
 <p>
 تحدثنا وقالت في لغتها: 
 « I should be living at 2, Atlas street</span> »
 ثم ذهبت. 
 </p>
 <p>
 تحدثنا وقالت في لغتها: 
 « I should be living at<br> 2, Atlas street</span> »
 ثم ذهبت. 
 </p>
</div>

note <br> before 2 in the second <p>.

Can CSS line breaking spec/rules consider such case (or similar)? I didn't see that in the document.
Or simply indicate in the first para of section-5 (https://drafts.csswg.org/css-text-3/#line-breaking), that forced break and soft break occurring at the same point, may have differences in rendering for some scripts (or something like).