You are indeed correct, a study by Karthikeyan et. al. suggests that the Dipole moment of terephthalic acid is indeed around ~2.6D (in air), the same as your latter source.
From the paper,
The calculated dipole moment of
the title compound is 2.57 Debye in gas and 3.14 in solvent
The same paper later mentions,
The calculated value of dipole moment is found to be 2.733
Debye. The highest value of dipole moment is observed in the component of µZ which is -2.733
D since the dipole vector is oriented negative Z direction. The dipole moment is zero in µX and
This is expected since terephthalic acid has only two stable rotamers with practically equal energies, both of which are planar.
Now even though the paper does not mention this clearly anywhere, what this suggests to me is 2.5704 D is the average Dipole moment of the 3a rotamer (varying from 2.4078 to 2.733 due to COOH twisting vibrations? unclear from the paper), since the Dipole moment of 3b should definitely be zero.
Since both the rotamers exist in nearly equal concentrations, This could suggest that the average dipole moment of should be written as ~2.6/2 D? It's unclear.
Karthikeyan, N.; Joseph Prince, J.; Ramalingam, S.; Periandy, S. Electronic [UV–Visible] and vibrational [FT-IR, FT-Raman] investigation and NMR–mass spectroscopic analysis of terephthalic acid using quantum Gaussian calculations. Spectrochim. Acta, Part A 2015, 139, 229–242. DOI: 10.1016/j.saa.2014.11.112.
Zoran, Marković; Dalibor, Bajduk; Ivan, Gutman ; Geometry and conformation of benzenecarboxylic acids, Journal of the Serbian Chemical Society 2004 69, 877-882. DOI: 10.2298/JSC0411877M