I don't understand this - asking @dnzxy to take a look.

The current dev branch has rexml under Gemfile.lock

• specs: CFPropertyList (3.0.7) as rexml
• specs: google-apis-core (0.11.3) as rexml
• specs: as rexml (3.2.9)
• specs: xcodeproj (1.24.0) as rexml (~> 3.2.4)

After making this modication, Gemfile.lock rexml is found under these locations (first 4 are identical, last is new):

• specs: CFPropertyList (3.0.7) as rexml
• specs: google-apis-core (0.11.3) as rexml
• specs: as rexml (3.2.9)
• specs: xcodeproj (1.24.0) as rexml (~> 3.2.4)
• DEPENDENCIES: as rexml (= 3.2.9)

And Gemfile has gem "rexml", "= 3.2.9" added (not found in Gemfile before).

And main branch is at rexml (3.2.6)

@dnzxy gave it two thumbs up.
I tested it with Mac-Xcode and Browser Build.
Success.