Created attachment 184716 [details]
.kdenlive file for referenced project.

SUMMARY

Saving a preset for fade out results in the fade out effect being applied at the point in time based on the clip on which the preset is created rather than the existing clip.

For example, I have two clips, the first of which is 00:00:08:18 in length and the second of which is 00:00:13:18 in length.  If I set a fade out time of 00:00:01:00 using the first clip, and apply that preset to the second clip, the fade starts at 00:00:07:18 of the second clip.  At 00:00:08:19 of the second clip, the fade abruptly goes away and the video returns to the original brightness

STEPS TO REPRODUCE
1.  Create two clips, clip A and longer clip B
2. Apply fade out clip to clip A of e. g. 1 second, and save it as a preset
3. Apply fade out to clip B, and apply the named preset from step 2

OBSERVED RESULT

The fade out in clip A is correct.  However, in clip B, the fade starts at the same place it would have started in clip A and ends at the length of clip A.

EXPECTED RESULT

Fade starts 1 second from the end of clip B

SOFTWARE/OS VERSIONS
Linux, using flatpak 25.08.0.

ADDITIONAL INFORMATION

Reference attachment.

From the XML file
clip A:
 <chain id="chain2" out="00:00:08.575">
  <property name="length">258</property>
...
  <filter id="filter14" in="00:00:07.608" out="00:00:08.609">
   <property name="start">1</property>
   <property name="level">00:00:00.000=1;00:00:01.001=0</property>
   <property name="mlt_service">brightness</property>
   <property name="kdenlive_id">fade_to_black</property>
   <property name="alpha">1</property>
  </filter>
 </chain>

clip B:
 <chain id="chain3" out="00:00:13.580">
  <property name="length">408</property>
...
  <filter id="filter17" in="00:00:07.608" out="00:00:08.609">
   <property name="start">1</property>
   <property name="level">00:00:00.000=1;00:00:01.001=0</property>
   <property name="mlt_service">brightness</property>
   <property name="kdenlive_id">fade_to_black</property>
   <property name="alpha">1</property>
  </filter>
 </chain>

From the preset file, we see that the time stamps are listed from the start of the clip (which only works when applying the effect to another clip of exactly the same length).

    {
        "One second fadeout": [
            {
                "in": 0,
                "max": 0,
                "min": 0,
                "name": "out",
                "out": 258,
                "type": 0,
                "value": 258
            },
            {
                "DisplayName": "Fade to Black",
                "in": 0,
                "max": 0,
                "min": 0,
                "name": "level\nalpha",
                "opacity": true,
                "out": 258,
                "type": 7,
                "value": "1\n0=1;-1=0"
            },
            {
                "DisplayName": "Duration",
                "in": 0,
                "max": 0,
                "min": 0,
                "name": "in",
                "opacity": true,
                "out": 258,
                "type": 14,
                "value": 228
            }
        ]
    }

The workaround is to not use presets for the fade to black filter, but this makes presets useless for this filter in almost all cases.  There is of course no corresponding problem with the fade from black (fade in) filter.