I'm less than convinced that Legendary Super Saiyan users can't initially stay in their Legendary Super Saiyan forms for long without fear of self destruction.
After all Broly spent a while in his Legendary Super Saiyan form and didn't explode.
Perhaps its present only in the manga and only for U6 Saiyans.
Legendary Super Saiyan Weakness
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Legendary Super Saiyan Weakness
"We became like friends, we became like good friends." Broly to Cheelai and Lemo about his fur pelt.