By default a character zoning into legacy raids will have it set to personal. You have to change this setting yourself. Since you are joining the raid solo the option to change loot isn't there. It's not FORCED personal loot, it's just the default setting.

All you have to do, as others have suggested, is open premade group, start a group, set group to private, and then just change from personal to group. From this point forward, your character will have this setting "saved"

There is no forced personal loot, that only exists for current dungeons and premade current raids or LFR.

As others have said, a legacy loot option will appear in BFA so you don't have to do the group finder fix anymore. Until then, you'll need to do it manually.