I saw a similar complaint about Bixby in the Samsung Members forum. I tried it out with my phone (Bixby) and it woke up fine. On those occasions when it doesn't work, it's because I'm in a noisy environment (my dump truck). I've resorted to launching my assistant by long-pressing my Galaxy Buds. fwiw
One of the reasons I hate Google Assistant, every time it updates OK Google quits waking up my phone, has happened to me for years on Samsung devices. Bixby is just another example of a Samsung app that works better than Googles.