Connecting blocks don't upgrade properly with custom dimension types

[Pablete1234]

When upgrading a 1.8 world with fences, iron bars glass panes, etc, paper does not connect them except on chunk borders:

This only happens when using a custom dimension type, if it is left unchanged (minecraft:overworld) then it does connect them.

The dimension type is being modified to a custom type to keep the min world height at 0 to keep the maps' gameplay the same (ie: no building under y=0). Blocks not connecting seems to be the only thing broken due to this, it is otherwise working fine.

Please note, i'm not totally sure if this is a bug or intended behavior. If it is intended that non-default dimension types won't get their connections updated, any hints, tips, workarounds, or pointers as to why this is like it is would be appreciated.

For reference, the map itself can be found in https://github.com/PGMDev/Maps/tree/master/The%20Fenland (level.dat + region folder only)