Too much "they/them" talk just looks poor, grammatically speaking. I mean sure, it makes sense in some context with the story, especially because we only got tiny glimpses of a person with a ponytail (Men can sport those too, don't deny it!), but it still just reads poorly. I mean no disrespect or shade, but that just sounds awful from an English standpoint. I assume the times "Master" were used only occurred when the translation had it spoken, but this is where a few liberties would be 100% viable. Hell, I remember having points taken off of my book reports because I used "they/them" too often when describing a singular person. That was an incredibly dumb reason to drop my A to a B, but still; poor grammar is poor grammar. Again, I mean no disrespect to the translator(s?). I'm thankful for the chapters since more Suoh is a blessing, but the grammar stuck out quite a bit despite being intentional.
Also, while I'm fairly inexperienced with all the gender stuff going on suddenly in the world, I do wanna throw this two cents out there; if Mute was referred to as female (and I totally imagined a nice motherly voice every time), then that was referring to the person and not the dog. Master may have been so flat that there might've been some concave action going on (especially with necrosis that bad, and dear god I hope not because that'd probably be awful), but if the person's been called one thing, stick to it.
Also also; Master is an octopus. Suoh's master is a biomechanical cephalopod now!