uneven Qi charging of iPhone 12 Pro

I bought an iPhone 12 Pro from T-Mobile in late 2020, which I keep at bedside as an alarm clock and music player/white-noise generator. (Also a 12 Pro Max from Apple in 2021, using AT&T, which is the phone I carry with me and charge with a cable). I used to charge it with the Lightning dock on my bedside clock-radio, but I noticed over time sometimes the battery would drain even when the phone was secure in the dock. I had an AppleWatch 4 that sometimes wouldn't charge on its bedside stand (which had a Lightning dock for my AirPods Pro) if it was the slightest bit askew. 3 months ago, I replaced the watch with a Series 7, and bought a Belkin 3-in-1 wireless charging pad from Apple. I notice that even though the watch charges quickly & consistently, and the AirPods remain charged, sometimes the phone loses charge--even though the little lightning-bolt symbol says it's connected (like what happened when I charged it in my clock radio's dock). It can be 100% one day and suddenly 40% and dropping the next--and I haven't moved it a millimeter. It sometimes takes a forced restart, even two to get it to charge rather than drain. I can only surmise the defect is in that model of iPhone, not the charge disc on the pad (because the battery drain occurred when plugged into the clock radio's Lightning dock). I've been blaming Belkin, but I think the fault lies with Apple (software, firmware or battery itself).