Craft:
probe core, power, central fuel tank (e.g. cdt2505) ready for LH2 and empty (may need to upscale to have enough space).
2 (or more) symmetry, add X11 to the central fuel tank, and upscale to 1.5x (doesn't happen on 1x).

Launch, select all 3 (or more) fuel tanks and move all the hydrogen to the central fuel tank, emptying the X11s. When the X11s reach a certain level, the craft is obliterated and sometimes parts will achieve relativistic speeds.

KSPIE 1.19.2

I think this happens because X11 have negative mass on upscale. This can and will create WEIRD things.
Changing tweakscale type = stack_interstellar_275 to type = stack fixes the issue.

Problem should already be fixed