SUMMARY
Plasma 5.15.0, 5.15.1, 5.15.2 Task Manager Sequence & Thumbnail bugs: Popup thumbnails have lost their VD#, & for VD-1 to VD-n, any windows using VD-n show first not last in Task Manager sequence. 

STEPS TO REPRODUCE
1. Upgrade Plasma to at least 5.15.0.
2. Activate Plasma Panel's Task Manager [TM], configured to arrange in Desktop sequence, Ungrouped.
3. Activate at least two Virtual Desktops [VD]. The following example uses 9 VDs, & the malfunction is confirmed also with 4 VDs [so i expect probably any number will suffice].
4. Open random program / document windows distributed across multiple VDs, except NOT the final VD [ie, use 1 - 8].
5. Inspect the TM & note that all its iconised windows correctly appear in the expected sequence.
6. Open another window, but now in VD9 [or alternatively drag an existing one there]. Reinspect the TM.

OBSERVED RESULT
1. VD9’s window/s appear NOT last in the TM, but incorrectly at the start [LHS-end if panel is horizontal] of the TM… ie, before even the genuinely VD1 windows.
2. TM no longer identifies the VD# in the popup thumbnails [this is unrelated to VD9 being occupied or not].

EXPECTED RESULT
1. VD9’s window/s should appear last in the TM sequence [which was true for Plasma <5.15.0].
2. TM window popup thumbnail should continue indicating VD# [which was true for Plasma <5.15.0].

SOFTWARE/OS VERSIONS
Operating System: Manjaro Linux 
KDE Plasma Version: 5.15.1
KDE Frameworks Version: 5.55.0
Qt Version: 5.12.1
Kernel Version: 4.20.12-1-MANJARO
OS Type: 64-bit
Processors: 8 × Intel® Core™ i7-4790 CPU @ 3.60GHz
Memory: 30.8 GiB of RAM

ADDITIONAL INFORMATION
1. See my thread in Manjaro forum https://forum.manjaro.org/t/plasma-5-15-x-task-manager-bug/77238?u=kdemeoz for more information & pictures. 

2. These bugs were NOT present in 5.14.5. 

3. These bugs [when Plasma >= 5.15.0] occur in [at least] Manjaro [all three branches], KaOS, & openSUSE Tumbleweed.