Well, I gave it a try (i.e., fixing problem via iTunes), and it did not work. Let me add a couple of more facts. My iPhone is an XS Max, IOS 15.4.1. I also have an iPad Pro, IOS 15.4.1. iTunes is version 12.12.3.5.
I plugged my iPhone into my computer and opened it under iTunes, as suggested. There was a Tones in the summary, but no ringtones were displayed (same with iPad Pro). No synch button either. On my iPhone and iPad Pro, under Sounds, my purchased ringtones were displayed. I once again hit "Download All Purchased Tones" on both. I selected one purchased ringtone for a default.
Next, I assigned specific ringtones to specific contacts via the edit & done process (which is why I bought multiple ringtones). Everything seemed fixed. The next day, all the ringtones on my iPhone reverted to the default (a purchased ringtone). The ringtones on my iPad Pro remained intact. The annoying thing is that, when I get a message from one of my selected contacts, I get two different ringtones (a default on my iPhone and the selected ringtone on my iPad Pro). So, an obvious question is what is going on with my iPhone that is not going on with my iPad pro, both using the same IOS (at least not yet)?
It is very surprising that this is still an issue since all the products are Apple related - iPhone, iPad Pro, iTunes. For some reason my iPhone is unable to retained the ringtones I have assigned. Maybe this was never possible in the first place with my IPhone.