Thanks. I've linked a .ptc exhibiting the problem.
I've also attached some videos showing the behavior in the editor using version 2.25 and 3.34.
Note that in 3.34 then opacity value is still interpolating on the timeline, but the actual image doesn't change its opacity.
PTC File
https://1drv.ms/u/s!AjpWSfu6NF23ieMTgSnS0TYBL6vxRgWorking in Magic Particles 2.25
https://1drv.ms/v/s!AjpWSfu6NF23ieMUh83XS5DkMA04tgOpacity not having any effect in Magic Particles 3.34
https://1drv.ms/v/s!AjpWSfu6NF23ieMSC28gjuU3rTnmRAthanks for your help
Andrew McDonald