Operating system: Linux-3.10.0-957.10.1.el7.x86_64-x86_64-with-centos-7.6.1810-Core 64 Bits
Graphics card: GeForce GTX 1070/PCIe/SSE2 NVIDIA Corporation 4.5.0 NVIDIA 418.56
Broken: version: 2.81 (sub 4), branch: master, commit date: 2019-09-02 15:37, hash: rB283d96de1170
Short description of error
Use modifier stack checkbox deletes all the hair particles if they were created with the add hair brush and combed (not necessary), even if there are no modifiers before the particle system. This also happens if particles have been deleted (X) in particle editmode.
And there is no way to get those hairs back. It can really destroy all your work on grooming.
By the way, that check box becomes inactive in 2.79 after grooming, but it's active for some reason.
Exact steps for others to reproduce the error
Create a hair particle system.
Go into particle edit mode.
Delete all the hairs (by the way, you won't be able to do that until you comb it, may be a second bug).
Add some hairs via add hair brush (you can add hair without deleting on the previous step, the bug will occur anyway).
Go back to object mode.
Check the use modifier stack checkbox.
All the hairs disappear!
The attached file has all that done except the very last step.