Step one - transmute something for 150 CP (fewer glyphs glyph, sensation stone).
Step 2 - transmute something else worth 150 CP - the bouns on your next conversion (noted in the portrait icon) goes down to 100% from 150%.
It's really odd. I'll try to recreate it. What is supposed to happen when you transmute two things without converting something in between?
EDIT: Just recreated and it's EVEN STRANGER.
Take a tinker with sensation stone into venture cave, prep fewer glyphs. Use one transmutation seal on the sensation stone (or a glyph). The status buff should say 150% bonus on your next conversion. Then pick a glyph up and use transmutation seal on IT. The item dissapears and the bonus on next conversion now sayes 100%. And if you convert something then (i converted a compression seal) - nothing happens.
So I can't tell what happens at all. If they were supposed to stack then it'd be either 300% or 225% on your next conversion. How it ends up at 100% is beyond me, but it's bugged even at that value. It's a rare occurence tbh, but weird, and what is supposed to happen if you try stacking transmutation bonuses isn't written anywhere...
I almost got pwned by Shifty Brickwork!