RPM fluctating after muffler delete

I have a 2018 Civic hatchback sport that I recently did a muffler and resonator delete on. Love the sound but I noticed that when accelerating or deaccelerating I get RPM fluctuations and they can be often sometimes extremely dramatic as far as 400 rpm differencing. Its stable during idle. Why is this happening? I've been reading a lot how others didn't have issues with doing this - does my car need to be tuned? From what I've read on various sites is I don't need back pressure, so what's the deal?
Thanks for any help.
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Despite what you may have read, I'm not buying it. Perhaps some cars are less affected than others, but exhaust systems on modern cars are carefully engineered to work in conjunction with all the other emissions and fuel delivery systems.