Black doesn't wrap long line but wraps shorter line

[DanInSpace104]

Describe the bug

Black fixes line with length of 120 but doesn't fix line with length of 152. When i tried to fix it myself Black reverted my changes.

To Reproduce

1. Take this code:

def temp():
    datetime_start_result = f'{datetime_start[6:10]}-{datetime_start[3:5]}-{datetime_start[0:2]} {datetime_start[11:]}'
    datetime_finish_result = f'{datetime_finish[6:10]}-{datetime_finish[3:5]}-{datetime_finish[0:2]} {datetime_finish[11:]}'

2. Run Black on it with these arguments -l 100 -S
3. First line with length 120 was fixed but second line with length 125 wasn't

def temp():
    datetime_start_result = (
        f'{datetime_start[6:10]}-{datetime_start[3:5]}-{datetime_start[0:2]} {datetime_start[11:]}'
    )
    datetime_finish_result = f'{datetime_finish[6:10]}-{datetime_finish[3:5]}-{datetime_finish[0:2]} {datetime_finish[11:]}'

Expected behavior
Black should format this code as:

def temp():
    datetime_start_result = (
        f'{datetime_start[6:10]}-{datetime_start[3:5]}-{datetime_start[0:2]} {datetime_start[11:]}'
    )
    datetime_finish_result = (
        f'{datetime_finish[6:10]}-{datetime_finish[3:5]}-{datetime_finish[0:2]} {datetime_finish[11:]}'
    )

Environment (please complete the following information):

• Version: 21.9b0
• OS and Python version: Linux/Python 3.9.5

Does this bug also happen on main?
Yes.

Additional context

When you format this code Black wraps both lines correct:

datetime_start_result = f'{datetime_start[6:10]}-{datetime_start[3:5]}-{datetime_start[0:2]} {datetime_start[11:]}'
datetime_finish_result = f'{datetime_finish[6:10]}-{datetime_finish[3:5]}-{datetime_finish[0:2]} {datetime_finish[11:]}'

I think this happens because black cannot split f-string into parts and so it can not make line shorter than 100 symbols, so it leaves this line unchanged. But in that case, Black should NOT discard my manual changes to this line and make it longer, or black should try to make line as short as possible even when it still longer than --line-length

[vbarbaresi]

There is an experimental flag --experimental-string-processing that solves this issue.
It will become the default behavior in the future #2188

[DanInSpace104]

There is an experimental flag --experimental-string-processing

Thank you, now it works just as I expected!