When a redmatter explosive detonates while being sucked in by another red matter java will basically quit responding and the following error will be spammed in the log:
https://pastebin.com/i6ZNi1W7
Provided that the game doesn't crash for long enough that error will be replaced by this error:
[17:40:10] [Server thread/ERROR]: icbm.classic.content.blast.BlastRedmatter@52166a95: Unexpected error running blast
java.lang.NullPointerException: null

The best way to reproduce this that i could find is to detonate a redmatter explosive and then detonating another nearby but making sure it gets stuck behind some blocks near the first one as it tries to suck it in so it detonates instead of getting sucked in.

ICBM version: ICBM-classic-1.12.2-4.0.0b20200101191300.jar
Full log:
latest.log

It's also worth noting that upon reloading the world that the second red matter explosive that had detonated next to the first one had disappeared.
EDIT: Upon doing this again the world had rolled back to before the detonations aside from 1 of the explosives being gone