Why does the GOOOOAAAALLLL! badge disappear after some time?

Probably you deleted the goal which earned the badge.

Some badges require to keep a thing / an activity / in this case a goal which completed them.

All targets are still in my GC, so that's not the case.

Have you tried editing one of the activities that was in the goal?

I've lost the GOOOOAAAALLLL! badge several times as well, but this last time it seems to stick. The goal I created was a really simple one just for the purpose to get the goal badge and make it stick. Log 1 activity in 1 week.

I didn't edit any of the activities. I'm sure I still had it last week, so had it for about 60 days/2 months after I got it.

I suggest that you should edit one of the activities now to get the badge back. I've heard that it might work, but I haven't tested it myself.

TY, I got it back by editting that feb. 18th activity. Hope this time it will last longer 

Mine is gone too, did one in January 2019 still with the Fenix 5, goal and activities are still there.