----------------------- 12/08/2018 -----------------------
It's a very annoying bug, I managed to reproduce it with my DataPack, it's really weird.
if you can take a look it would be nice.

Bug : When I disconnect and then reconnect display of the bossbar no longer works,
but the values are always there.

When I do "/bossbar set levelup player1 visible false" and "/bossbar set levelup:player1 visible true" display works again, but it's pretty weird anyway.

To Reproduce :
1. I managed to reproduce the display bug with my datapack so I advise you to install it to realize this bug
Download LevelUp

2. -Create a world with the Datapack /reload if needed
-You will see a bossbar at the top as in the screenshots.
(I configure them to increase alone)

3. You just have to log out and log in to see the display bug.
if you want to try again /function levelup:reset and /reload

I'm sorry for spelling I'm french I use google translation

----------------------- 13/08/2018 -----------------------
I find more information about this bug,

So to reproduce the bug :
1.Create a BossBar and configure this.

/bossbar add bug {"text":"Bug Tracker"}
/bossbar set minecraft:bug players @s

2.Create a scoreboard

/scoreboard objectives add Time minecraft.custom:minecraft.play_one_minute
/scoreboard objectives add Max dummy
/scoreboard players set @s Max 2000

3. Create a Animate Bossbar, In a Repeat commands blocs

execute store result bossbar bug max run scoreboard players get @p Max
execute store result bossbar bug value run scoreboard players get @p Time

4. log out and log in to see the display bug.