mrot not working properly when save has different ID mappings than inital settings

[HamiltonKsos]

I created a mrot file for backpacks! and tinker's construct and it worked fine on the test world I had to create it, but when I use a previously saved world, the rotations did not work, and the helm would not rotate correctly. Vanilla block rotations were still working correctly, except for the mysterious disappearance of some ladders, which may be unrelated. I did not edit the default .mrot file.

After reviewing the FML logs I found these relevant entries:
[08:33:26] [Client thread/TRACE] [FML/backpacks16840]: Registry add: backpacks16840:backpack_block 165 brad16840.backpacks.blocks.BackpackBlock@14c10311 (req. id -1)
[08:33:26] [Client thread/TRACE] [FML/ArchimedesShips]: Registry add: ArchimedesShips:marker 169 net.minecraft.item.ItemBlock@1af3bf5b (req. id 169)

During Initialization block numeric block ID's are mapped.

[08:33:40] [Client thread/TRACE] [ArchimedesShips/ArchimedesShips]: Adding meta rotations (block=ArchimedesShips:marker, id=169, mask=3, rot=[0, 1, 2, 3])
[08:33:40] [Client thread/TRACE] [ArchimedesShips/ArchimedesShips]: Adding meta rotations (block=backpacks16840:backpack_block, id=165, mask=7, rot=[3, 4, 2, 5])

Mrot files loaded following the initial block ID's

[09:16:07] [Server thread/DEBUG] [FML/]: Fixed item id mismatch backpacks16840:backpack_block: 165 (init) -> 795 (map).
[09:16:07] [Server thread/DEBUG] [FML/]: Fixed item id mismatch ArchimedesShips:marker: 169 (init) -> 165 (map).

on world load, the numeric block ID's change. I verified with Waila, and the metadata is following the backpack rotation sequence.

[Yuri6037]

I got the same bug rotations are invalid, but with the Marker block (Ship's Helm).

I'm using the latest GitHub on my IDE...