This is a standard 20 x 20 mm calibration cube (from thingiverse) but scaled height to 60 mm in Cura 2.6.2.
Vase mode. printed to test different temperatures. The problem occurs even with stable temperature as the shortest cube shows (that one was printed at 215 C)
Retraction was ticked on at 25 mm/s. But I don't think it retracts in vase mode anyways, right?
The problem gets better with more height as the cubes show (bases on right, tops on left of image).