Usually yes - sometimes the gate attendant doesn't understand that the exhale is filtered (I assume this was the problem with your respirator). We have guidance on that here:
I'm not even one to argue with people in these situations so I was happy to play along but that attendant seemed eager to escalate.
Perhaps having the extra filters like you recommend might help, but I suspect it wouldn't have. Hope things have improved since then (a month ago on Alaska).
1) so scared of their management that they won't listen to people explaining the masks are filtered or
2) simply unable to understand that a non-standard mask could have a filtered exhale valve (which is much safer than a cloth mask for others, too).
I had the exact same thing happen to me on 4 different flights, with the flight attendants becoming very aggressive. I brought a short note with a diagram of how the filtration works, and pointed it out to them, and they just told me "sorry, Mayo Clinic guidelines say you can't wear that." This was on United, American Airlines, and Delta.
The tip I figured out on my last flight was to wear a hoodie that covered up the respirator on the side, and then wear two surgical masks over the respirator in front. Haven't gotten a second look since then. I don't think this would be possible with the narwhal mask (unfortunately).
A friend of mine postulated that they couldn't hear me through my mask, but as I took it off and was swapping I said "I'm complying, I'm complying, I'm not arguing" in a conciliatory tone, it didn't seem to help. I was playing the 100% doormat. Didn't stop the attendant from saying to me as I was leaving to go to my seat: "It's not that dangerous".
I can only guess that they encounter people who do insist or that they are on web forums / chat groups with other attendants who have encountered people who do argue and so they're primed for someone like that. Puzzling interaction, to be honest. It was the first time I've had any sort of negative interaction with a service professional.
One gal even made me take off the mask and put on a surgical mask (which she gave me, she wouldn't let me wear my non-vented N95) in front of a security camera. That was pretty embarrassing, to be holding up the line, and felt like a scene out of a dystopian comedy.
I can only reason that it's for liability reasons, and the airlines don't want to be sued for someone getting infected.
But then again, people right behind me had cloth masks on with their noses exposed, who obviously didn't care, and they didn't get a second look. Neither did the airlines keep the middle seats free like they had promised too. Clown world.
Once I was on the flight I put my respirator on with a hoodie over it and slouched against the window, and they didn't bother me.