This actually has been around since last season and I don't think is a bug, but more of an oversight.
The perk description says it increases the duration of sun spots, not the duration of healing. So you can actually re proc the healing by walking in and out of the sun spots. The hunter duskfield gauntlets had a similar issue but I believe it got fixed eventually.
So hopefully they will extend the duration of healing while wearing this exotic.
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BagzxxYes, it is me, Bagzxx - old
In fact, no, perk description says what "Sol Invictus lasts twice as long", not "restoration", so this is broke of perk on legs, because normal longness of Sol Invictus is 5 seconds, when with legs 10 (5×2=10) seconds To understand, Sol Invictus is an aspect of Solar Titan, and perk fron this aspect give you faster recharge of abilities and your super lasts longer when you enter the sunspot AND also gives you restoration