Muscle does not weigh more than fat - a pound is still a pound. Muscle is more dense, so you may show fewer inches for the same weight if you are more muscular, but weight can still be the same.
Few can actually gain muscle mass while losing total weight, but we can seek to lose as little as possible while losing the fat. Typically we will lose some muscle mass, particularly in the legs and core, as we don't need as much muscle to support and move our reduced weight around, and few can dedicate the time and effort required to build up a comparable amount elsewhere (and then keep up the extra work to maintain it so that it doesn't turn to fat over time.)
For the OP, yes get a body composition scan to see where you are - Dexa is considered the best, but water or air displacement or even the scales give useful information if used properly. As a woman, mid 20's body fat percent is typical healthy range, but individually, you might be better as 22-23%, or 27%, so don't obsess over it.. I aimed for mid teens, which is mid to lean normal for men, and called it a day. Even Dexa isn't accurate enough to obsess over a point or two one way or the other.
To get back on track, as you have acknowledged falling back into old habits, try just going back to basic lean meat and green vegetables for a while. What is typically needed is a few days of "detox" from the (typically sugary) junk that has crept back in to alleviate your cravings for them.
These liquid "reset" diets don't do anything better and just reinforce whatever tendency you may have toward yo-yo dieting. They are premised on the observation that when we first had our WLS we were typically on a liquid diet for a while and we lost like gangbusters, so therefore if we go back to a liquid diet, we will lose like gangbusters again. Unfortunately, it was not the liquid diet that caused us to lose rapidly, as even those of us who never did a liquid diet lost like crazy those first couple of weeks. It was all about the heavy caloric deficit that we were in courtesy of our surgery, and the resulting water weight loss as our body adjusted to the new reality.
In short, cut out the junk that has crept back in and replace it with high nutrition, low calorie real food that will satisfy you nutritionally and cut your cravings for sugar, and is something that you can stick to in the long term, rather than a quick fix yo yo diet.
Good luck,,,