I had this exact problem on my iPhone X. It went on for a few weeks and then my phone refused to charge.
I do do think that it is caused when the phone has reached 100% and stopped charging; and then starts again. So not beeping when it reaches 100%, but rather when it dipped below 100% and starts charging again.
I had it replaced at the Apple store. After that, the new phone was fine.
until yesterday when it started beeping again on the charging pad.
I set it up as a new phone, but I have re-added many of the apps that were on the previous phone.
its doing it again with many different charging docks, cables, etc.
So unless I just happened to get 2 bad hardware phones, I think it IS software related - possibly in how IOS works with some 3rd party Qi chargers due to how they start and stop... or possibly due to specific apps that we have installed.
Don’t know if this helps any; but I do know that declaring with certainty what the problem is based just on just personal experience and the berating others who think otherwise surely does not.