Bug in the "by adding" record rule option?

I think something has changed (in the last few…days? weeks?) with this type of record rule:
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We’ve only ever used the “by adding” option. (Which implies that any existing values will be retained; the rule will “add” some value(s) to any existing values. We’ve used this rule successfully this way for some time.) Our understanding of the “by adding” function is further clarified by the existence of the “by replacing with” option, which sounds self-explanatory. I don’t have any experience with the “by removing” option.

Our issue is that the “by adding” option is not working…instead it behaves like “by replacing with.” I can go into our use case if that would be helpful - and I’ve already submitted a ticket - but I thought I’d ask if anyone else has had an issue with this feature or can confirm?

The only clarification I should probably give is that this is used on a form that creates a new record, and the rule is used to “update this record.” Happy to fill in more blanks for anyone reading!