When using scarab brooch according to old wowhead comments it should be stacking with the Val’anyr buff however when I use this trinket the brooch will not go on someone with a Val’anyr bubble. See this old thread about 1/2 down someone says it does indeed stack with Val’anyr. https://www.wowhead.com/wotlk/item=21625/scarab-brooch#comments There are also multiple other forums confirming it does indeed stack with Val’anyr.
Tons of the stuff in “Classic” is broken and Blizzard effectively does zero to fix it unless you play a class that the Dev’s are presently playing.