Steps to Reproduce:
1) Run 

/scoreboard objective add fireworklife dummy
/scoreboard objective setdisplay sidebar fireworklife

2) Run in a repeating command block

/execute as @e[type=minecraft:firework_rocket] run execute store result score @s fireworklife run data get entity @s Life 

What the above does is that it shows when fireworks:

• are loaded when the score of the firework goes up
• are unloaded when the score of the firework freezes
• are completely gone when the firework and its score are deleted from the scoreboard

3) Give yourself fireworks of duration 3 and an elytra

4) Set gamemode to adventure mode (which makes it easier to use the firework)

5) Go to the end portal (in the end to overworld where the dragon dies) and point your camera at an angle so that you are not facing the obsidian pillars or the middle of the end portal  This is to prevent the firework colliding with these blocks.

6) Fly and propel yourself with the firework into the end portal

7) You should see that the firework entity remains on the scoreboard with its score frozen

8) Go back to the end and see that the firework's score goes up and can either disappear or freeze

9) If it freezes even in the end, you can run 

/execute as @e[type=minecraft:firework_rocket] run data get entity @s 

10) Teleport to the firework by shift clicking the "Firework Rocket has the following..." to see that it has flown into unloaded chunks