After setting up my system newly (OH 2.5.1 via openhabian on a Raspi3) I copied the saved ,rrd files from the old setup and changed the onership back to openhab:openhab.
Looking into the rrd4j database via the REST API I see the saved old datapoints, the new datapoints and the timegap when nothing was saved is visible as well.
When graphing the data with the standard sitemap graph however I only see the actual value presented a line as if this value was stored for the whole selected time (on any selected timeframe, D, W, M).
Looking at the same data with a Habpanel Chart (n3-line-chart) I do see all values correctly!
See these examples for both chart types:
I am actually out of clues how such is possible.