promql: Fix wrongly scoped range vectors

Fixes #11708.

If a range vector is fixen in time with the @ modifier, it gets still
moved around for different steps in a range query. Since no additional
points are retrieved from the TSDB, this leads to steadily emptying
the range, leading to the weird behavior described in isse #11708.

This only happens for functions listed in `AtModifierUnsafeFunctions`,
and the only of those that takes a range vector is `predict_linear`,
which is the reason why we see it only for this particular function.

Signed-off-by: beorn7 <beorn@grafana.com>

promql/engine.go

@@ -1494,10 +1494,14 @@ func (ev *evaluator) eval(expr parser.Expr) (parser.Value, annotations.Annotatio
 						otherInArgs[j][0].F = otherArgs[j][0].Floats[step].F
 					}
 				}
-				maxt := ts - offset
-				mint := maxt - selRange
-				// Evaluate the matrix selector for this series for this step.
-				floats, histograms = ev.matrixIterSlice(it, mint, maxt, floats, histograms)
+				// Evaluate the matrix selector for this series
+				// for this step, but only if this is the 1st
+				// iteration or no @ modifier has been used.
+				if ts == ev.startTimestamp || selVS.Timestamp == nil {
+					maxt := ts - offset
+					mint := maxt - selRange
+					floats, histograms = ev.matrixIterSlice(it, mint, maxt, floats, histograms)
+				}
 				if len(floats)+len(histograms) == 0 {
 					continue
 				}