Do you have a source for the inscription?
Guillaume Dye and Manfred Kropp, “Le nom de Jésus (ʿĪsā) dans le Coran, et quelques autres noms bibliques: remarques sur l’onomastique coranique,” in
Figures bibliques en islam, eds. Guillaume Dye and Fabien Nobilio (Bruxelles: E.M.E & Intercommunications, 2011), p. 182, with reference to Antonin Jaussen and Raphaël Savignac,
Mission archéologique en Arabie II/1 (Paris: E. Leroux, 1914), p. 232, n° 386 (see also p. 228, n° 370).
Arab Christians do not pronounce the name that way.
By that I simply meant that they pronounced it as such in their
popular speech, as the form Yaḥyā might be a distortion of Yuḥa(n)nā, just like ʿĪsā.