That's harsh. Definitely can't recommend replaying the whole game again just for them to bug out again. Personally, after some time off I was in a mood for another playthrough anyway and specifically focused on bounty hunters right off the bat, so I managed to get them scanned and got the platinum later.
What I did for this to work:
1. Started a new game on easiest difficulty.
2. Skipped all out of the way collectibles (I got them earlier anyway) and focused on scanning all enemies.
3. Got captured in the ice caves by a big robot bounty hunter (went through Imperial Dig Site by vines) and scanned it after the big fight club fight.
4. After being rescued from the fight club I immediately uploaded my save files to online storage for backup in case this bug appeared again (I'm on PS4).
5. Went to Bogano and saved at the meditation spot before the door to the Abandoned Workshop and went back and forth to the cave where Oggdo Bogdo boss was until the flame throwing flying bounty hunters appeared there and scanned them.
6. Went to Kashyyk and from the starting point used the horizontal elevator to get to the Imperial Refinery and then immediately after that used the elevator to go down to a room where the last type of bounty hunter spawned and scanned him.
7. Proceeded to complete the whole game because the last enemy appears just at the end of the game.