MR targeted a commit instead of into master

Per the #is-this-known channel, I am submitting an issue to investigate an error of merging MRs that have multiple changes (commits) and targeting commits. The steps I typically take are:

1. make additional changes to an open MR in the browser by editing in WebIDE
2. stage & commit new changes
3. commit to the existing branch to (see image, red square).

After it is merged, the result is it targets a commit instead of the master. Here is a recent MR where this happened. Any information would be appreciated, thanks!