Your flat is caused by the tube ballooning down into below the constricted area at the bead into the well of the rim. It happens at the valve, or more precisely near the edge of the reinforced area.

If you look at mounted tire in cross section you'll see that the space necks down at the bead, then there's the area in the rim's well beyond that. On a narrow rim than necked-down width is very narrow, and if the valve cannot pull down enough, it keeps the base above (below in picture) allowing the inflated tube not only to expand down into the well, but to expand back to fill the gap at the valve. That's more than the rubber can tolerate so that little side bubble bursts.

The solution depends on the details. If possible, pull or wiggle the valve until it's out of the rim as far as it can go without breaking, as a last stem after mounting and seating the tire. If that doesn't work, because the rubber cone on the tube is too wide, consider using a tapered reamer to slightly enlarge the valve hole. (deburr the sharp edges when done)

Another option is to consider changing to Presta valves whose, narrower stems are more suited to narrow rims, and can slip between the beads more easily. That opens a few other issues, so if you opt to go that route, search PV in SV rim on this forum for more info (you might have to write out Presta and Schrader), but you'll cross that bridge if/when you come to it.