There seems to be an ongoing bug with the Concentration API available in the mod library that causes an error lock bug when used with a character who has proficiency in constitution saving throws (or who has a non-proficiency manual adjustment to their CON saves).
It also seems like "sebastien g." flagged this back in July to no response. https://app.roll20.net/forum/post/11563141/concentration-api-bug
Has anyone found a solution or workaround for this issue?
Not sure if I'm just screaming into the void here, but I think I've learned more about the cause of the bug and have found a workaround.
The cause: something in this script seems to break if a character's "constitution_save_bonus" attribute reaches double digits. The earlier suspect - that it broke if a character is proficient in the save - only seems to matter if the total saving throw mod reaches 10+.
The workaround: I'm disabling auto-roll and will make the Tank character with a 10+ constitution save roll his saving throws outside of the macro. That way the other members of the party can still use the API roll, and even the Tank can benefit from the script's reminder functionality.