See PM-3050 – streaming group is one of the optimizations, added in that project. It replaces $group with $_internalStreamingGroup if the input into the group is sorted on time and the group’s key includes a monotonic derivative of time.

$_internalStreamingGroup isn’t supported in SBE and while the query could benefit from lowering the prefix up to it, we’d rather not risk perf regressions and block it from running in the hybrid mode until $_internalStreamingGroup is implemented in SBE.