Although the red matter appears to save correctly(It still exists after reloading a save etc) it does throw this error in the logs: https://pastebin.com/64ZuCYzF

Used ICBM version: Self-build until this commit: 2a19265

This may have been fixed in commit 9c6db8e
Could you re-test it, including this commit?
(stacktrace is very similar to #346)