"Locate features along routes" tool and line features - how to use XY tolerance?

I am trying to use the “locate features along routes” tool to map guardrails to a route, but I’m seeing some very strange behavior. As a test case, I’m selecting a single guardrail (pictured) that is about 8 meters from the route.

If I set the XY tolerance to 5m, it works perfectly and gives me a single record in the resulting event table with a from and to measure. But if I set the XY tolerance to 10m, the resulting event table is empty. The docs say this is a cluster tolerance when line features are used, so why would 5m work but 10m not? I’ve tried several values:

2m = doesn’t work

3m = works

4m = works

5m = works

6m = works

7m = doesn’t work

10m = doesn’t work

What is causing this? I’m trying to process thousands of records so playing with each individual guardrail’s XY tolerance is not going to work.