Just a note; this may also apply to pumkins as well.

I was playing a survival world in the RTX beta (not on an RTX world, with cheats off) and planted a mellon seed I got in my starter chest. I used bone meal on it untill the stem was fully grown and went mining. When I came back, the mellon hadn't grown from the stem yet (it had been about 20 minutes). I thought this was odd considering I didn't go far at all from where it was growing, so I left my charater afk for about an hour right by the stem (about five blocks away). When I came back, the mellon still hadn't grown.