hi @hellocatfood - thanks for reporting this issue you are observing. having never used a mainbow myself i cant be sure exactly why this is happening, however here is the previous post mentioned about b/w issues with mainbow output into v8:
although this is other way around i guess could be mainbow having trouble locking on to the recurboy sub-carrier…
one thing you could do is test putting something else (like a distribution amp or tbc) between rpi and the mainbow to see if that makes a difference.
it could be that this issue is unique to pal if other users are reporting these two working together ok - so could be worth testing both with ntsc if possible.
finally another (slightly round-about) option could be to take the hdmi output from recurboy and convert this to composite with an hdmi->composite adapter and see if this would solve your problem
i would like to build a mainbow one day (and am also in PAL land) so if i ever do will be able to test this for myself also