I only used a solid under the transmission once. For some reason it cracked the tail housing at the mount. I was told at the time to never use a solid mount on the engine and solid under the gear box.
When I switched to an engine plate and mid mount, I solid mounted the gear box and never hurt it. Never figured out why the first one broke.
Seems like a no-brainer to not use a solid at the trans.... once the frame starts flexing, then the force concentrates more there. I am more curious about why you did not break the trans end with the plate in place....