Not Attached particle system is not seen in the viewport of the editor. This particle will be seen normally when PIE.
Regression? Yes. This issue is not reproduced in 4.17 version.
You can simply check using attached project. You will see Red and Green circle (particles) when playing level sequence, but you won't see red one(Not attached particle) when you open this project in 4.18.2 version.
1) Add ThirdPerson template
2) Create a particle System
3) Open AnimSequence'/Game/Mannequin/Animations/ThirdPersonRun.ThirdPersonRun'
4) Add Play Particle Effect notify
5) Set new particle system for new notify
6) Set Attached to false
7) Set Socket Name as "head"
8) Go back to viewport of the editor.
9) Place a Mannequin skeletal mesh(SkeletalMesh'/Game/Mannequin/Character/Mesh/SK_Mannequin.SK_Mannequin') into viewport
10) Create a new level sequence
11) Add the skeletal mesh into sequence track (AnimSequence'/Game/Mannequin/Animations/ThirdPersonRun.ThirdPersonRun')
12) Save all and reopen the project
13) Open the level sequence
14) Play the sequence in the editor
Expected: the particle system is visible
Result: the particle system is not seen.
12) Add new animation (
There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-53617 in the post.