Hey I just had the same issue with my wife’s trainer tire.
The problem was that the tire has cracked around the perimeter from heat/cool cycles and the tube was pushing into those cracks and puncturing. I had to replace the tire and that fixed the problem. I didn’t realize it the first time I replaced the tube because I didn’t look close enough or pinch the tire in my fingers to see if it was cracked.
Look around the sidewalls and see if you have cracking like the photo below:
