I just got around to trying out the new 1.13.1 version. When I tried to make a superflat preset with the deep lukewarm ocean biome and strongholds enabled, strongholds would not generate. Figured maybe this was an issue with the biome, but after trying a dozen different configurations (all containing the stronghold generation option in the preset code), they all didn't generate any Strongholds. /locate Stronghold would always say that there was no stronghold nearby and Eye of Ender just did nothing when used.

This happens even with the "Overworld" preset, which is clearly meant to contain strongholds as it includes the stronghold generation option in its preset code. So to reproduce:

1. Create a new superflat world with the "Overworld" preset
2. Try /locate Stronghold and/or using an Eye of Ender
3. Observe that no stronghold can be located

Strongholds work just fine in normal worlds, so this issue appears to be specific to superflat. They would also generate fine on superflat in 1.12.2, meaning this issue was introduced by the new version.