Sequencer - When a subsection has negative play rate, any nested sub-subsections fail to evaluate

Consider a sequence hierarchy with more than two layers. If the top-level sequence uses a negative timescale on a sub-sequence, the contents inside it animate as expected, but any nested sub-sequences fail to evaluate and are silently ignored (spawnables are not spawned, possessables are not possessed). This is true regardless of how the sub-sequence section's negative timescale is applied - with a fixed play rate, a play rate curve, or a time warp curve.

In UE 5.3 and 5.4, the negative timescale attempts to evaluate negative frames and fails even on the first subsequence level. Attempting to turn on "can loop" freezes the editor.

In UE 5.5 and 5.6, the negative timescale results in top-level frames being converted to (range-frame) inside the subsequence where the setting is applied, and its contents are animated correctly. However, other subsequences nested within it are completely ignored. This is true whether "can loop" is on or off (actually, enabling it even creates an odd behavior on the first frame).

===== BLANK PROJECT =====

1. Create a new level from the Basic template
2. Add 4 cubes to the level
2.1. Set X = 300, 500, 700, 900
2.2. Set Y = 0
2.3. Set Z = 49.5 (ground level)
3. Create and edit a new Level Sequence asset "LS_SubSub"
3.1. Add cubes 1 and 2 to the Sequence
3.2. Convert cube 2 to spawnable
3.2. For both cubes, on Frame 0, add location keyframe Y = -250
3.3. For both cubes, on Frame 150, add location keyframe Y = +250
4. Create and edit a new Level Sequence asset "LS_Sub"
4.1. Add a subsequence track for "LS_SubSub"
4.2. Follow steps 3.1 to 3.3 with cubes 3 and 4
5. Create and edit a new Level Sequence asset "LS_Main"
5.1. Add a subsequence track for "LS_Sub"
5.2. Scrub the timeline between frames 0 and 150. All 4 cubes should move together.
5.3. Right-click the subsequence section – Properties – Timing – Time Scale – Change from 1.0 to -1.0
5.4. Scrub the timeline between frames 0 and 150. The first subsequence is evaluated correctly, but the nested one is not.

===== REPRO PROJECT =====

The project contains a level with 3 cubes:

• Red: Possessed and animated by sequence "LS_SubSub"
• Green: Spawned and animated by sequence "LS_SubSub"
• Blue: Spawned and animated by sequence "LS_Sub"

It also contains the following Level Sequence assets:

• "LS_SubSub" animates red (possessed) and green (spawnable) cubes
• "LS_Sub" has a subsequence for "LS_SubSub" and animates blue (spawnable) cube
• 6 top-level sequences, each with a subsequence for "LS_Sub"

The top-level sequences are meant to repro the issue with different "Time Scale" settings on the subsection:

• 01: Plays forwards with a fixed play rate of 1.0
• 02: Plays backwards with a fixed play rate of -1.0
• 03: Plays backwards with a Play Rate Curve set to constant -1.0
• 04: Plays backwards with a Time Warp Curve that linearly interpolates from end to start
• 05: Plays forwards then backwards using a Play Rate Curve set to 2.0 then -2.0
• 06: Plays forwards then backwards using a Time Warp Curve that linearly interpolates from start to end and back

On PIE, the level blueprint plays the 6 top-level sequences one after the other:

• 01: works correctly
• 02-04: the blue cube controlled by "LS_Sub" animates correctly, but the other cubes controlled by "LS_SubSub" are not possessed/spawned.
• 05-06: works correctly in the forward animation, incorrectly on the backward animation

The level blueprint also has a "PlayReverse" flag to play the top-level sequences in reverse mode, which does not help.