Operating system: Linux-4.15.0-55-generic-x86_64-with-debian-buster-sid 64 Bits
Graphics card: GeForce GT 540M/PCIe/SSE2 NVIDIA Corporation 4.5.0 NVIDIA 390.116
Broken: version: 2.80 (sub 75), branch: master, commit date: 2019-07-24 14:22, hash: rB507ffee6e1f4
Short description of error
Operator has effect only after tweak "last operation options" (F9)
Exact steps for others to reproduce the error
Select default cube, enter mesh edit mode, select all faces, subdivide it once.
Select any corner vertex and move it a little to produce asymmetric shape.
Select all vertices and perform "Snap to Symmetry" operation (nothing happens because threshold is to low).
In the 'last operation options' pannel set 'Threshold'=1m, now the shape became symmetry.
Repeat some steps:
Move any corner vertex again, select all verts and perform "Snap to Symmetry" operation. Nothing happens.
Tweak any param of the operation (F9) and notice that the mesh is changed at this moment.