Like "SMRS", I am having the same sort of problem.
I created my ringtones in Premiere Pro. Exporting the ringtones to "Stereo AAC, 44.1 kHz 128 kbps" then renamed the file to m4r. Once my IPhone 7 was connected I made sure "Manually Manage Music and Videos" was selected and applied. I then tried dragging and dropping the file to the tones section (sidebar), the tones page, my device in the side tabb and also tried Command+C the m4r files and then right clicking and pasting (making sure it was actually pasting instead of Command+V) it into the tones section. Nothing, no response. I then went back and exported my ringtone as "Stereo AAC, 48 kHz 256 kbps", renamed to m4r, repeated all the steps trying to move/paste the files into my Tones. Still nothing. I tried exporting one of the files through a converter online to m4r (I know you don't need to convert to m4r from an aac file I was just trying) still didn't work. I have restarted both my phone and laptop as well as made sure that the m4r filed I was trying to transfer had read and write access granted on all users.
Iknow these files aren't DRM protected as I created them.
To further "SMRS's" point of it maybe having something to do with my iPhone file structure being write protected. I used a third-party software (WALTR 2) to move the m4r files to my phone, these transferred over. However, once in the phone's settings>sound>ringtone I clicked on the files and it would just play the default sound. Now this could be a transfer issue but I also transferred over ringtone I made before the 12.7 iTunes update using the app and the ringtone worked. Now knowing the app works I made sure my exports I had made had sound when playing the original AAC files, they worked fine. As soon as I changed the extension to m4r my laptop can't play the file. I assume this is because of my laptop not recognising the file as its now a ringtone.
Also with the m4r files, I transferred over using the 3rd party app, finding out they didn't work I wanted to delete them. I connected my phone back up to iTunes and could see the m4r files (transferred through the 3rd party app) in my tones. selecting the ones that didn't work, I deleted them from the library and then synced my phone. The files no longer showed up in my tones library on my laptop but the names were still present under my settings in my phone, still not working.
This is why I think it is either the iPhone file structure being write protected or iTunes having a problem with m4r management, between iTunes and my phone or just iTunes itself.
I have no idea where to go from here or how I can go about trying to acquire my personal m4r/ringtones on my IPhone 7. Any help would be appreciated.