The bug
After the recent change in 1.15 snapshots, firework rockets aren't oriented properly when fired from upwards or downwards facing dispensers.
Firing rockets from hand still works fine.
Examples
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Linked issues
is duplicated by 4
MC-188015
Incorrect model display of firework_rocket when fired from a dispenser facing y-axis
Resolved
MC-177488
Fireworks launched from vertical dispensers fly facing sideways and away from the player
Resolved
MC-170543
Firework rocket entity tilt is not suitable for heading when launched from dispenser
Resolved
MC-172130
Dispensing a firework with a dispenser facing upwards will summon a sideways firework
Resolved
Affects 20w21a