Punching with a cast fishing rod in the off-hand detaches fishing line from rod

Can confirm in 21w03a.

At net.minecraft.client.render.entity.FishingBobberEntityRenderer.render() (Based on 21w05b yarn mappings) The fishing bobber gets the hand that is currently holding a fishing rod in it. It then uses the main hand's swing progress to determine the fishing line's animation when punching, which is currently used as part of the animation regardless of which hand is holding the fishing rod.

int j = lv.getMainArm() == Arm.RIGHT ? 1 : -1;
ItemStack lv6 = lv.getMainHandStack();
if (!lv6.isOf(Items.FISHING_ROD)) {
      j = -j;
}
float h = lv.getHandSwingProgress(g);
float k = MathHelper.sin(MathHelper.sqrt(h) * (float)Math.PI);

This could be fixed by checking if the hand with the fishing rod is the same as the main hand, and if not then setting h to 0:

int j = lv.getMainArm() == Arm.RIGHT ? 1 : -1;
int j2 = j;
ItemStack lv6 = lv.getMainHandStack();
if (!lv6.isOf(Items.FISHING_ROD)) {
      j = -j;
}
float h = j == j2 ? lv.getHandSwingProgress(g) : 0;
float k = MathHelper.sin(MathHelper.sqrt(h) * (float)Math.PI);

Blocked from testing in 21w11a, by MC-219849