I don’t think the controllers will play the effect unless they’re being used. If you weren’t wearing the HMD (proximity sensor) and/or the controllers were sleeping(?) I wouldn’t be surprised if it’s not played.
I wouldn’t use the Grab Action Input event as a test for haptics on the Index controllers as that’s not working consistently - I responded to another post about that here: OpenXR Valve Index - #2 by VictorLerp
Oh and I should mention that there are no more updates to 4.27, but if the problem exists in UE5, provide repro steps and I’ll get the issue logged. Thanks!