Growing world borders reset to their starting size upon reopening the world

I have been playing with a slow growing worldborder for a long time, but noticed some strange setbacks on my border for some time now.

Here is the most simple test case I could find:

• Create a new world with commands allowed

• /worldborder set 15

• /worldborder add 60000 5000d

• Wait a somewhat long time, until the border reaches size 20 will show the problem well enough

• Take note of where the border is

• Save and quit to title

• Load the game again : the border is around size 18

I have tried a lot of different growth speed and numbers with /worldborder add, but it seems that the main factor is the time spent in game. If I quit and load after a short time playing, the border seems to stay where it was. But the more I spend time playing, the bigger the setback when I quit and load again.