I have been studying these "lumps" in the bettas for over 10 yrs now, and I still have found nothing that can be done for it. They are not tumors... I have done a number of autopsies and disections, and it is for sure not a tumor. I can't find any glandular issues in these fish, either, but I have noticed that many of them have issues with their immune systems and tend to get sick much quicker and easier than the others. Whatever it is that causes this, it's commonly found, and nothing treats it. If you notice your fish beginning to lose color, lose a lot of weight and sort of "waste away" I would suggest humanely euthenising it. Once they begin to waste away, it can take months, but they steadily go downhill and you can see that they are suffering. When selecting a new betta, this is something to look for when choosing a healthy fish. It is possible that this is caused by something genetic...
Sorry I don't have better news to post for you. Good Luck to you and your fish!
Mine has it too ever since I got him from petco. He is super playful, swims actively, is always hungry but i strictly give him 4 pellets a day to maintain his diet or 3 frozen fruit flies. I think this is not an underlying disease infact on youtube betta owners have some with these too yet they claim their bettas lived up to 7 years even though petco says they can only live up to 3. I have a thermostat to keep the water at 78 degrees and change 100% spring water/filter per week (although its not tap water i still mix water conditioner 10 drops per gallon), clean the gravel + decors per week, test the ph water quality weekly at a light blue which is perfect. I had to get him out of the small miserable petco bowl. Compared to others he was the only one that followed me so I knew he was the one. It's not tumor but could be excess or swollen muscle tissue because hes always swimming which means hes happy but it should reduce over time if they get some rest lol.