What you’re describing is Exercise Adaptation, or, “training plateau”. It’s a common response to exercise stress. From the National Academy of Sports…
The principle of adaptation refers to the process of the body getting
accustomed to a particular exercise or training program through
repeated exposure. As the body adapts to the stress of the new
exercise or training program, the program becomes easier to perform
and explains why beginning exercisers are often sore after starting a
new routine, but, after doing the same exercise for weeks and months
at the same intensity, the exerciser experiences little, if any,
muscle soreness. This reinforces the need to constantly vary the
exercise and training routine if you want to maximize your results.
However, each of us react differently to the stress. Some may reach a training plateau where gains are few or non-existent. While others, with the same stress, can continue to make improvements. That’s why it’s important to recognize a plateau and make the necessary changes to continue improving. The key is to always challenge your body. That may take the form of increasing the intensity, or, changing the order of exercises, sets, and reps. In my experiences, once a plateau has been reached, I try to take a week off to recharge and refocus my efforts. My goal is to come back with a fresh approach to my training.
So, to answer your question, adaptation is real and should not derail your training progress if you recognize it and make the appropriate changes.