When you use the /tick rate command, then save and exit your world, relogging will result in the tickspeed being reset to normal.

To replicate:

Create a new singleplayer world with commands on

Run the command /tick rate [number that is not 20]

Save and quit to title, then play the same world.

If the number from earlier was close to 20, you may need the command /tick query to see that the tickspeed has been reset.