I can confirm this - the TR seems to be displayed incorrectly in fslhd if set to zero:
me@christop ~/Downloads $ nib-ls -H pixdim zerotr.nii.gz
zerotr.nii.gz int16 [ 64, 64, 21, 200] 3.12x3.12x6.00x0.00 [1. 3.125 3.125 6. 0. 1. 1. 1. ]
me@christop ~/Downloads $ docker run -ti --rm -v 'C:\:/c' bids/base_fsl
root@9495a8535534:/# fslhd /c/Users/me/Downloads/zerotr.nii.gz
filename /c/Users/me/Downloads/zerotr.nii.gz
sizeof_hdr 348
data_type INT16
dim0 4
dim1 64
dim2 64
dim3 21
dim4 200
dim5 1
dim6 1
dim7 1
vox_units mm
time_units s
datatype 4
nbyper 2
bitpix 16
pixdim0 0.000000
pixdim1 3.125000
pixdim2 3.125000
pixdim3 6.000000
pixdim4 1.000000
pixdim5 1.000000
pixdim6 1.000000
pixdim7 1.000000
vox_offset 352
cal_max 0.0000
cal_min 0.0000
scl_slope 1.000000
scl_inter 0.000000
phase_dim 0
freq_dim 0
slice_dim 0
slice_name Unknown
slice_code 0
slice_start 0
slice_end 0
slice_duration 0.000000
time_offset 0.000000
intent Unknown
intent_code 0
intent_name
intent_p1 0.000000
intent_p2 0.000000
intent_p3 0.000000
qform_name Scanner Anat
qform_code 1
qto_xyz:1 3.125000 0.000000 0.000000 -98.039986
qto_xyz:2 0.000000 3.125000 0.000000 -105.085075
qto_xyz:3 0.000000 0.000000 6.000000 -57.780876
qto_xyz:4 0.000000 0.000000 0.000000 1.000000
qform_xorient Left-to-Right
qform_yorient Posterior-to-Anterior
qform_zorient Inferior-to-Superior
sform_name Scanner Anat
sform_code 1
sto_xyz:1 3.125000 0.000000 0.000000 -98.039986
sto_xyz:2 0.000000 3.125000 0.000000 -105.085075
sto_xyz:3 0.000000 0.000000 6.000000 -57.780876
sto_xyz:4 0.000000 0.000000 0.000000 1.000000
sform_xorient Left-to-Right
sform_yorient Posterior-to-Anterior
sform_zorient Inferior-to-Superior
file_type NIFTI-1+
file_code 1
descrip FreeSurfer May 22 2011
aux_file