Well, sorry for not using murmur?
There is some testings involved. The point of Testing (and Unstable) is to see if incoming updates goes well or not and (when it is possible) fix issues before it goes on Stable. Volunteers on Testing and Unstable receives updates, upgrade their system as usual, and report issues they find if there is any issue on their systems, on their hardware+software configuration. And it does help, because Manjaro was close to publish in Stable a pretty broken version of a package, but because it just happened that one guy used that package and saw how f- up it was, we could fixed the issue before it goes on Stable.
But what happen if no one, absolutely no one in Testing/Unstable use that package in their daily use? Well, no one see if there is regressions or not. Therefore, the more people with different hardware+software configuration goes on development branches, the more software we can cover.
If you think we (people in Testing and Unstable, which would maybe nearly 100 people at best active on this forum if I check the pools in announcements) can cover all single combination of hardware+software on a distro that has like more than 10 000 packages, even the most obscure ones, you might have to revise your expectations a little bit honestly.
The question is if the fixed from Arch Linux got published before or after May 22nd, 2019, because the last big update set we got in Testing that updated packages imported from Arch Linux was published in May 22nd, 2019; if the fix was introduced in, let's say, May 23rd, 2019, well it appeared one day too late to be on Testing at that moment.
According to this ticket, it was fixed in May 23rd, 2019, therefore one day too late compared to the last big update set in Testing. That's why it wasn't introduced in Stable even if it was fixed before Stable received a big update release.
According to the logs, it is a rebuild against grpc 1.21. Only the pkgrel got bumped, with no changes in the build file.
Also, "I'm curious how a broken package managed to get into Arch Linux Stable, I thought there was testing involved?"