simply put the soul elytra doesn't give the boost while flying. And after 10 minutes of constant flying i think something is broken
to replicate just use it

doesnt boost as in automatically boost? or it doesnt work with fireworks

doesn't work automatically. works fine with rockets. did you discontinue it ? i checked and it is the same in fabric 1.20.2, but in mod showcases it is show to work(1.19)

yea we removed the feature because some people liked it and some didnt. we are currently developing an alternative to the automatic boost

oh, hope it goes well, good luck.