Vertically Stacked Dimensions: https://www.curseforge.com/minecraft/mc-mods/vertically-stacked-dimensions

Attempting to ascend from a lower dimension while using a morph with a significantly lower view height than the player often results in the player being immediately teleported back into the lower dimension. A player with a one-block height (morphed into a silverfish, parrot, etc) will also teleport when standing on the lower of the two layers of purple bedrock/portal blocks, so I assume what is happening is that the player is being placed on a lower bedrock/portal block and instantly teleporting back to the lower dimension.

I assume this is a result of either how Changeling achieves its lowered player view height, or a result of Vertically Stacked Dimensions using the position of the player's view height (rather than their feet) to decide if they are low enough in the world to teleport to the lower dimension. I reported this on Vertically Stacked Dimensions' issue tracker as well.