That's another problem with memory comparisons: I don't know what gear I had back then, though I never had colours -- 'dun use that one on any build. Might have had exhaustion, too (via rose-thorn).
However, I recall stay-strong as the tipping point; ie. adding it to whatever else I may have had flipped the slow from useless to useful.
Aye, I forgot about Exhaustion and that would add to that.
But that's OK, let me go dive into another test, see if the Improved Precise Shot (IPS) triggers on multiple, if so, we know that you're missing some other stuff that would have proc'd or stacked on top to line up enemies. If it (IPS) doesn't work, we'll know from the test I do in a moment.
J1NG
:: edit ::
Preliminary testing suggests that you can proc Stay Strong on multiple targets, but they need to proc the chance for it individually. So in a line of 5 enemies, each one has a % chance of getting Stay Strong Slow effect, rather than one procs all. Once the other Tendon Slice effects work again, stacking the other effects that reduce the speed of the targets will make it easier to replicate what you experienced before.