When using send_event, if you omit the sequence or leave it empty, Minecraft hard crashes. If you have "base_delay" and/or "event" it doesn't crash.
In 1.1 you could omit the sequence and filter to use the particle effect. Presumably not defining a filter and event should save resources?
I could understand if event was required, but the fact that it works even if you only define base_delay (with no event) doesn't feel like it's working as intended.
I attached a working and broken skeleton.json for testing.
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