Per https://groups.io/g/GroupManagersForum/wiki/6321/61677
it appears that threads with dissimilar subjects will be merged if the prefix “Re:” is recognized. We have a member that has additional labels added to their email subject on reply that foil automatic thread merging:

[Ext] [<groups.io group name>]

[Ext] Re: [<groups.io group name>]

I propose that in addition to “Re:”, any labels of the syntax “[.*]” be added to this current algorithm. If a regular expression were used then it would not depend on the content within the brackets, merely any subject with brackets would be ignored.