Very likely the cause for MC-187475
The outside planes of fire are completely vertical in Java Edition as of 1.8. However, from Indev up to 1.7, they would have a gentle inwards slope. This change was unnanounced, hence why I wish to confirm this as a bug. The slanting would also allow some inner planes to intersect the outer planes andbe visible this way, which is far less noticable on the current model.
This is also a parity issue, as Bedrock Edition still appears to have the old model.
Fixing this may require adding new allowable angles for rotation in model files.
Can confirm in 21w38a.