flip state not stored in tmj format

Describe the bug

Ive transitioned to using tmj format for ease of editing and use. Ive hit a big where the flip states are not stored in the data. Its adding a gid: field, but it doesnt change the value.

Steps to reproduce

1. add a template object. save it.
2. now flip it. save it.

Compare the maps are identical. Loading it back in tiled and it never saves the flip state.

Expected behavior

flip states need to be saved. Without it i cant flip objects in my game.

OS version

linux

Tiled version

1.12.2