It's a process breakdown, either manual or automated. Marketing teams tend to use a tagging worksheet like this[1] when creating new emails. They're just told to put all links through that process, and the topic of tel links don't really come up. I've handed off dozens of link generators like that to marketing and account management folks, and never thoguht to add any checking for tel links in the logic or to surface that edge case in the instructions.
So if they're going through the motions they were told to, but don't fully understand why (and don't QA properly) this is what you end up with.
Could also be a system using the same logic of surfacing every <a> tag link for tagging before deployment (based on the utm names, there's at least some type of semantic logic to the naming that indicates a person was involved at some level and knew it was a phone number they were tagging).
So if they're going through the motions they were told to, but don't fully understand why (and don't QA properly) this is what you end up with.
Could also be a system using the same logic of surfacing every <a> tag link for tagging before deployment (based on the utm names, there's at least some type of semantic logic to the naming that indicates a person was involved at some level and knew it was a phone number they were tagging).
[1] https://docs.google.com/spreadsheets/d/1xDl9GdpuWB9kzXzWxKj3...