As I already stated, reordering midi for a given time stamp is fair game. So yes pianoteq is reordering those events. This is correct behavior for a midi device.
The steam 5371 is maybe different but is similar. You need to do playback prep before any notes are processed. In this case you need to assert the articulation events before time 0 as that is the time stamp of the first note.
This midi ordering within a plugin is no different than midi across several staves or devices. Those can be processed in any order as long as you respect the time stamps.