Path of Burning Steps is supposed to grant you an increasing bonus to Solar damage, up to x 4.
You can get up to x 3, but when you would get to x 4, you instead get nothing. The buff icon goes away. Your damage resets to base damage. You can't gain any more buffs from the exotic until you wait 10 seconds.
I'm guessing it's applying some invalid buff ID or something instead of Solar Damage Boosted x 4, and you have to wait for that 10 second duration to fully expire before you can get x1, x2, x3 again.
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Hi there, Thank you for your report. We are aware of players reporting this issue with Path of Burning Steps and are investigating.