Multi-line graphs should behave the same in a trellis or not for single-points

I have a survey that we've administered across multiple years to different school districts. One of the main topics of interest is how different roles have changed over time. Using a multiline graph in a trellis does a great job visualizing that for a district that has taken it multiple times:

However for a district taking it the first time, not so much:

When I take the multi-line out of the trellis, it behaves the way I would want it to, showing a point for values with only a single year:

The multi-line should behave the same way in the trellis as it does without a trellis. When I submitted a support ticket, they said this is the expected behavior. If it's the expected behavior, why isn't it consistent in a trellis/not in a trellis?

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