As Marksmanship Dark Ranger, zoning from a delve into Undermine will make it so Bleak Arrows no longer functions and are instead firing as physical damage Auto Shot. It seems specific to Undermine and specific to delves. Both the Underpin delve and the Sidestreet Sluice will cause the bug every single time I try it. I zoned and exited the Undermine raid and didn’t run into the problem.
Swapping to Sentinel and back to Dark Ranger will fix the issue until you enter and exit a delve in Undermine again. At which point you will need to swap to Sentinel and back to Dark Ranger to fix it.
I’m still doing some tests to see if I run into this problem while doing M+ or if it is exclusive to the delves in Undermine and the Undermine outdoor zone.
Slight update: simply getting in your D.R.I.V.E. vehicle and exiting will make Bleak Arrows no longer function while in Undermine and instead fire as physical damage Auto Shot.