I noticed this bug in 1.8 forge, but it is still in the 1.16.5 version I'm using. If an entity gets a scoreboard value and is then unloaded, the score will be deleted. The same effect does not happen in vanilla. It does happen no matter what mods are loaded.
You can reproduce this by:
Opening a world
Leaving the spawn chunks
/summon armor_stand ~ ~ ~
/scoreboard objectives add test dummy
/scoreboard players set @e[type=minecraft:armor_stand] test 1
/scoreboard objectives setdisplay sidebar test
At this point you'll see on the sidebar that there is an entity with a score of 1
Fly away until the sidebar disappears becaues the armor stand is unloaded
Return to the armors stand
The sidebar has not reappeared because the score is gone, you can also use the /scoreboard players get command to verify
If you repeat this in vanilla the sidebar will reappear when you return
Log: