I repeated the problem.
Download osuire’s files. Open File 02. Start a worksession and attach File 01.
TweenSurfaces on the two surfaces.
Result is a surface which is not on any existing (or at least visible) layer.
From What on the created surface.
surface
ID: 3207025b-fda5-4970-ad38-de2afc812291 (192)
Object name: (not named)
Layer name: File 01.3dm::Default
A slightly similar bug: Worksession Error - Copy InPlace