Hit a nerve? We’re you there perhaps?
The blast radius of this particular diner was at least 5 tables wide. Think some of the examples in the DYKWIA thread.

No guessing required. Facts are more useful I find.
In common with everything else this diner said to the staff that night, the approach to the tip was broadcast for all to hear. “No, I won’t sign for a tip. I will leave it in cash”. As soon as the waitstaff withdrew, a €5 was plopped on the table and said diner made a hasty exit.

More guessing? I never indicated either the gender or the nationality of the diner.

Up to the two critical queries in the final 15 mins of the meal (“please hurry up” and “why am I being charged €6 for coffee with petit fours”) every other broadcast commentary was on the lines of “the bread is wonderful, please could I have just one more piece”, “the fish is divine”, etc. etc. etc.

Avoiding eye contact was more the issue rather paying attention!

Happy to share more details (as in facts) if it helps sooth that nerve I hit.

My favourite tale about tipping (a little OT sorry, but perhaps a gentle distraction amidst this slightly touchy thread topic) concerns the late Francis Albert Sinatra : an individual known for being extremely generous & appreciative to all who waited on, helped, and / or generally pleased him, especially those in low-paid roles (albeit in stark contrast to the less pleasant treatment meted out - allegedly - by Sinatra’s ‘people’ to anyone who dared incur his wrath).

The great man once asked a parking valet what was the biggest tip he ever received. To which the reply came, "$100, Sir”
Sinatra - never one to be outdone - promptly proceeded to give him a tip that was considerably larger than that. Following which curiosity got the better of Sinatra .... so much so, that he demanded to know who was the guy who had tipped $100. At which point the grateful valet said, "It was you, Mr. Sinatra."

See my earlier responses to some of the above. Staff were nothing but professional and appropriate.

The total was verbalized loudly before the query on the cost of coffee.

12.5% would be standard local tip.

Anyway the point I was trying to make was that under-tipping (assuming you believe this is a thing) is not unique to visitors to the US

i know France pretty well indeed, one would tip to signify that they were really happy with the service (and nowhere near us level because as you rightly say, service has to be included by law as in most civilised countries so you just top it up really) and conversely you wouldn’t leave anything if you found service mediocre. However, every restaurant I know in France will have sharing arrangeants in place so it will be shared between your waiter, kitchen, bar etc.