"phantom" fill in timeseries

I have a timeseries, where some sort of “phantom fill” is resulting in a diagonal line.
See the pictures for an explanation. One graph shows the dots, the other the strage result when the dots are being connected.

It almost looks like there’s some sort of “invisible dots” causing this.

I’ve seen this in both 10.2.3 and 10.3.1

Generally, provide reproducible example.

For example:
1.) show full raw data (use table view):

2.) show all panel config options, not just some:

3.) show all overrides

BTW grouping per 30 sec is overkill, why you don’t use $__interval?