I wanted to provide an update on the solution to my problem as it might help others. The problem was that the phone charged extremely slowly or not at all, and didn’t show the lightning bolt icon when charging. I ordered a new bottom module and a new battery, and when they were delivered I replaced the bottom module and now the phone charges fine (with both the old battery and the new battery), and the lightening bolt icon now displays again when it is charging. So the problem was clearly the bottom module.