Likewise a refusal on a TPE service, where the guard couldn't process an excess fare, refused to do so and then wanted to issue a TIR. Eventually after quoting the cost of the excess and a 10 minute polite argument (a change of route on the return portion of a LDS-GLQ from via Appleby to Any Permitted) it was done but after being told I intended to evade the fare by not obtaining an excess before hand when there isn't a requirement to do so. This should be a simple easy to do matter for anyone with training on selling tickets.
I have purchased a few one-direction route excesses in the past. More of them have been got wrong than not, most commonly charging the full difference rather than half the difference, but also outright refusal and probably other screw-ups I can't remember. I don't know why so many staff seem to have issues with them - and unlike most other issues discussed in this thread, not knowing how to do them or being unwilling to do them is definitely, in my experience, a majority problem and not a minority one.
You've then got if it comes up as zero fare...they really run a mile then. I have
never managed to have a zero-fare excess issued, ever; there have always been excuses made or flat refusal - and I have tried a few times. Even where I've had something issued, they've done it another way - such as Euston LM ticket office preferred to refund a ticket that I had already partially used for travel and sell a replacement than to do a zero-fare overdistance excess. Back in the more reasonable days of the 1990s it'd just be "it's the same fare, don't worry about it", but now things are absolute this problem needs to be solved.
The problem with a TIR being issued is that if it gets to shysters like TIL (Transport Investigations Ltd[1]) then what they don't do is look at it in detail to see if it was right or not, but rather they just go straight for the jugular.
[1] So "professional" that their SSL certificate expired two days ago. I'm not convinced their response to someone whose season ticket expired two days ago would be as accepting.