Issue in stress calculation (LAMMPS 29 Sep 2021)

I was shearing the system with fix deform and wanted look into the stress-strain (only virial part of stress and compare it with non-virial part, just for confirmation) curve so I did the following :
variable XY equal "xy/ly"
compute 1 all pressure NULL virial

thermo_style custom step pxy v_XY c_1[4]
thermo 1

but this gave me the following output :
0 -0.00051075213 0 -0.00051895765
1 -0.00051074031 1e-05 -0.0005189712
2 -0.00058060248 2e-05 -0.00058887656

You see the starting two values of ‘‘pxy’’ and ‘‘c_1[4]’’ are the same. So this would imply that the stress value is constant of first strain value and then it starts to increase, which is unlikely. I firstly dumped the position and found the stresses from my own code and it was always increasing. Then I also did the following to reconfirm :

variable XY equal xy/ly
variable PXY equal pxy
variable SXY equal c_1[4]

variable deforming loop 5000
label loop

run 0
print "${PXY} ${XY} ${SXY}" append stress.txt screen no

run 1

next deforming
jump shear_twoatoms_earlier.txt loop

this gave me the following values :
-0.00051075212750827 0 -0.000518957654572431
-0.00058065158183846 9.99999999997137e-06 -0.000588882471504961
-0.000647824192077961 1.99999999999427e-05 -0.000656098269348236

The stress is increasing with strain, one interesting thing is that the stress for the strain value = 0 and 2e-5 from the earlier output matches with this output for strain value = 0 and 1e-5. Why is this happening?