I was filming a PVP sequence with my friend on a LAN world using 1.16.3 Fabric, Replay Mod, Optifine and Sildurs Extreme v1.281. My friend was using a crossbow charged with fireworks. Now, when rendering the sequence, whenever I go to render a clip or jump to a previous location (that happened annytime after the firework was shot) in the top timeline the creeper face shaped particles appear and will act like they are regular fireworks. None of the other particles that are part of the firework render. (they were red creeper, yellow and pink ball flight dur. 1)

Edit: It appears that a temporary solution is rendering an extra ~5 seconds at the start (to allow the particles to appear and fade) and then cutting that 5 secs with an external editor.

A screenshot after jumping through the timeline.

A screenshot of the rendered video playing in Windows Films & TV

This is related to #302 #311. The only temporary solution I know is the one you mentioned.

Might also mention that block breaking particles appear if a block changes at some previous point(seems to be known but, eh) and also clarify that the fireworks that were being shot had more than just a red creeper face, they had yellow and pink as well that wasn't rendering when it 'intialized', which could be another bug altogether.