It looks like that if the cursor isn't clipped by the application (or
by wine in fullscreen), then both master and slave XI2 device events
are received (on the contrary, when the cursor is clipped then for some
unclear reason only the slave device events are received).

In this case, one of the events is then discarded by
merge_raw_motion_events in dlls/winex11.drv/event.c and as
X11DRV_RawMotion is designed to track only slave devices (one per
thread with the xi2_current_slave field AFAICS?), depending on which
comes first, the event might not be processed and no raw input be sent.
--
Rémi Bernon <rbernon@codeweavers.com>