Artstyle changing and developping throughout a serie is pretty normal and common. The faces are leaner than at the beginning, but the overall artstyle is similar and the quality hasn't dropped significantly, so I don't think there's really cause for worry, imho.
That happens. You can look at the before/after for any long running series and the same applies. It would be odd for nothing to change in 2+ years tbh. I'm glad the quality hasn't dipped.
We should be worried when the background is no longer being drawn and colouring and line art get more patchy.
Although, it does feel a bit sad when that "sweet spot" was earlier in series.
I feel a little bad pointing it out since it doesn't overly bother me. Just couldn't help but notice how different the the recent illustrations/chapters look compared to the older ones, I might be insane
hope everything's alright behind the scenes though, if there really is a problem