The harmonics I’m getting are sympathetic. Usually I’ll play a line and notice them ringing out on the e string. Usually, they don’t stop unless I mute with my left hand or more firmly push my thumb into the string. So what I’m saying is my thumb will be “floating” on the e string yet it doesn’t prevent the e string sympathetic harmonics.
I don’t think floating thumb is a bad technique I just feel like it’s a bit of false advertising. Going into it I thought it would be the one stop shop right hand does all the muting but you still have to get your left hand involved. I don’t think there is any bass technique that can entirely mute. It’s always going to be a combination of right and left hand muting.
You probably just need to make a very minor adjustment with the position of your thumb. There are a lot of natural harmonics in that part of the string, but not all of them speak easily, so it should be matter of finding a dead space to rest your thumb, and then adjusting the geometry of your accordingly.
I am not aware of anyone who has said floating thumb was easy, or that there were not problems to overcome. Yes I agree, you need to use a combination of right and left hand muting techniques, especially if you play 6-string.
It's not just the thumb on my plucking hand. In fact I use all of my fingers except the thumb on my fretting hand for muting. At times I also use the outside of the palm of the plucking hand.
When I transitioned from 4-string to 5-string I did not notice any need to pay extra attention to muting. My 4-string technique seemed to work just fine. When I transitioned to 6-string, I noticed that I was often having significant trouble controlling all of the strings. I believe it took me about two years to really expand and automate my muting technique so I could play cleanly without having to think about it.
To this day, from time to time I need to stop and deconstruct how I am playing a certain line in order to control undesired string ring.
IMHO floating thumb is more effective than anchored thumb for muting, but it's not perfect. You still have to work things out and make adjustments from time to time.