In my case, the reason was that I had accidentally changed the building style for this district, which, without mods, results in Cities: Skylines deleting all the houses in quick succession until there are none left which have the wrong style. (After which the zoned area will slowly be settled as usual (but with different-style buildings), if there is enough demand.)
Changing the building style back will stop the rest of the houses from unspawning. In my case, the game was paused when I accidentally changed the style and I saved before unpausing, so I was able to go back to an older save, quickly pause it when it had loaded and change the building style back before the homes vanished.
In case you still want to change the building style
This mod lets you change the building theme for a district with existing buildings without despawning them. I haven't personally tested it, though.