As a maintainer, I can certainly understand how it feels like that, I'd probably wouldn't feel great about it either. As a user, I'm curious what kind of modifications they felt were needed, what exactly did they change in your library?
This was a while ago (10+ years), but my recollection is that someone presumably had reported that parts of the library didn't conform to the spec, and Debian patched those. This broke parsing actual feeds, and caused weeks of debugging issues that couldn't be replicated. Had they reported upstream in the first instance, I could have investigated, but there was no opportunity to do so.
Good intentions, but unfortunately bad outcome.
There was a somewhat recent discussion on here on how OS projects on GitHub are pestered by reports as well. Some athors commented that it even took away their motivation to publish code.
It’s always the same mechanism isn’t it. The „why we can’t have nice things“ issue. Making everything at least slightly worse, because there are people who exploit a system or trust based relationship.
However, I do believe that in certain areas, they give too much freedom to package maintainers. The bar for being a package maintainer in Debian is relatively low, but once a package _has_ a maintainer--and barring any actual Debian policy violations--that person seems to have the final say in all decisions related to the package. Sometimes those decisions end up being controversial.
Your case is one example. Package maintainers ideally _should_ work with upstream authors, but are not required to because a LOT of upstream authors either cannot be reached, or actively refuse to be bothered by any downstream user. (The source tarball is linked on their home page and that's where their support ends!) I don't know what the solution is here, but there are probably improvements that could and should be made that don't require all upstream authors to subscribe to Debian development mailing lists.
Debian has earned that trust, and it's software update rules are battle-tested and well-understood.
It's typically an unglamorous, demanding, unpaid, volunteer position a few rungs above volunteering at a soup kitchen or food bank. It's unsurprising that the bar is low.
It's also trivial for upstream maintainers to set up their own competing Debian package repos that entirely ignore Debian rules - Microsoft has one for VS Code.