Rendering bug in heatmaps?

When drawing a heatmap using data from meshping, I sometimes get a straight line at the highest data point like here:

That topmost line (in the 137ms bucket) looks somewhat bogus, and when I zoom in into the time frame, it vanishes:

Every time those lines vanish as soon as I zoom in to before their end, so to speak. The rightmost point is actually valid, but I see straight lines rendered all the way to the left border of the graph that shouldn’t be there, confirmed by the fact that they go away as soon as I zoom in. If they were valid, they should also be in the zoomed-in version.

Is this indeed a rendering bug, or am I doing something else wrong? Data source is Prometheus, and I’m using delta(meshping_pings_bucket{target=\"$target\"}[1h]) for the query.

This data is actually returned by my query… so something’s off here, but Grafana only makes it visible.