Merging after Subtractive Merge deletes files, instead of ignoring addition

[Kike]

In our project, one user merged by mistake a task branch (br1) into an unrelated one (br2). When they noticed the mistake, they performed a subtractive merge from the undesired merge changeset (br1 -> br2), which resulted in a changeset (!br1 -> br2) full of br1 files deletions and additions.

They kept working as usual, until recently we noticed a lot of files for a specific level are missing on main. After investigating the history, we've seen the following sequence:

br1 -> main: files added in br1 are now in main

br2 -> main: many of the files added from br1 are deleted from main, some files that were deleted in br1 reappear in main

It looks like the br2 -> main merge took the changes from the subtractive changeset and applied them. This is not the desired nor expected behaviour we want from a subtractive merge. The fact that a link is created lead us to believe that it meant "all operations should ignore the changeset this is coming from". But it is, however, a complete changeset in itself, leading to our issue.

Is there any way to prevent this from happening? What's the alternative to subtractive merges, given that deleting non-head changesets is cumbersome and we cannot select several files from a changeset and say "revert to this/previous version" in bulk?

Are there plans to fix this behaviour of subtractive merge?

We'll try to figure out a way to recover the files deleted and remove the ones added unintentionally. And, from now on and until subtractive changesets behaviour changes, we'll forbid anyone from using them.