Why does fastlane add_badge work for me locally but fail on App Center - fastlane

Question

As per the title, if I execute the following (for example) from my local machine, the command succeeds and all my icons get updated correctly:
fastlane addBadge branch_name:staging build_number:1558591886 branch_color:green
However, when the build runs on Visual Studio App Center, it errors with the following output:
[32mDriving the lane 'addBadge' 🚀[0m
[06:46:53]: [32m-----------------------[0m
[06:46:53]: [32m--- Step: add_badge ---[0m
[06:46:53]: [32m-----------------------[0m
[06:46:53]: [32mStart adding badges...[0m
[06:46:54]: './iOS/Assets.xcassets/AppIcons.appiconset/icon120x120.png'
dyld: Symbol not found: _OBJC_CLASS_$_NSFont
Referenced from: /Library/Frameworks/Mono.framework/Versions/5.18.1/lib/libpangocairo-1.0.0.dylib
Expected in: flat namespace
in /Library/Frameworks/Mono.framework/Versions/5.18.1/lib/libpangocairo-1.0.0.dylib
+---------------+----------+
| [33mLane Context[0m |
+---------------+----------+
| PLATFORM_NAME | |
| LANE_NAME | addBadge |
+---------------+----------+
[06:46:54]: [31mNo such file or directory # rb_sysopen - 1558593854-staging-green.svg.png[0m
For App Center, I'm calling the fastlane addBadge command from appcenter-pre-build.sh.
Question: Why is this error happening, what do I do to fix it?
Update 1: After adding --verbose to the addBadge command:
Here's what I get locally:
Driving the lane 'addBadge' 🚀
INFO [2019-05-24 09:58:54.78]: -----------------------
INFO [2019-05-24 09:58:54.78]: --- Step: add_badge ---
INFO [2019-05-24 09:58:54.78]: -----------------------
INFO [2019-05-24 09:58:54.86]: gem 'badge' is already installed
WARN [2019-05-24 09:58:54.87]: Install RSVG to get better results for shields on top of your icon
WARN [2019-05-24 09:58:54.87]:
WARN [2019-05-24 09:58:54.87]: Install it using (RSVG):
INFO [2019-05-24 09:58:54.87]: $ brew install librsvg
WARN [2019-05-24 09:58:54.87]:
DEBUG [2019-05-24 09:58:54.88]: Verbose active... VERSION: 0.10.0
DEBUG [2019-05-24 09:58:54.88]: Parameters: {:shield=>"1558591886-staging-green", :no_badge=>true, :glob=>"{/iOS/Assets.xcassets/AppIcons.appiconset/*.{png,PNG},/Droid/Resources/**/{icon}.{png,PNG}}", :dark=>nil, :alpha=>nil, :alpha_channel=>nil, :custom=>nil, :badge_gravity=>nil, :shield_parameters=>nil, :shield_io_timeout=>nil, :shield_geometry=>nil, :shield_gravity=>nil, :shield_scale=>nil, :shield_no_resize=>nil, :grayscale=>false}
INFO [2019-05-24 09:58:54.88]: Start adding badges...
DEBUG [2019-05-24 09:58:54.88]: Trying to load image from shields.io. Timeout: 10s
DEBUG [2019-05-24 09:58:54.88]: URL: https://img.shields.io/badge/1558591886-staging-green.png
INFO [2019-05-24 09:58:56.36]: './iOS/Assets.xcassets/AppIcons.appiconset/icon120x120.png'
DEBUG [2019-05-24 09:58:56.36]: Adding shields.io image ontop of icon
INFO [2019-05-24 09:58:56.43]: './iOS/Assets.xcassets/AppIcons.appiconset/icon58x58.png'
And here's what's outputted on App Center:
INFO [2019-05-24 00:13:28.42]: [32mDriving the lane 'addBadge' 🚀[0m
INFO [2019-05-24 00:13:28.43]: [32m-----------------------[0m
INFO [2019-05-24 00:13:28.43]: [32m--- Step: add_badge ---[0m
INFO [2019-05-24 00:13:28.43]: [32m-----------------------[0m
INFO [2019-05-24 00:13:28.50]: [32mgem 'badge' is already installed[0m
DEBUG [2019-05-24 00:13:28.51]: [34mVerbose active... VERSION: 0.10.0[0m
DEBUG [2019-05-24 00:13:28.51]: [34mParameters: {:shield=>"1558656638-staging-green", :no_badge=>true, :glob=>"{/iOS/Assets.xcassets/AppIcons.appiconset/*.{png,PNG},/Droid/Resources/**/{icon}.{png,PNG}}", :dark=>nil, :alpha=>nil, :alpha_channel=>nil, :custom=>nil, :badge_gravity=>nil, :shield_parameters=>nil, :shield_io_timeout=>nil, :shield_geometry=>nil, :shield_gravity=>nil, :shield_scale=>nil, :shield_no_resize=>nil, :grayscale=>false}[0m
INFO [2019-05-24 00:13:28.51]: [32mStart adding badges...[0m
DEBUG [2019-05-24 00:13:28.51]: [34mTrying to load image from shields.io. Timeout: 10s[0m
DEBUG [2019-05-24 00:13:28.51]: [34mURL: https://img.shields.io/badge/1558656638-staging-green.svg[0m
INFO [2019-05-24 00:13:29.27]: './iOS/Assets.xcassets/AppIcons.appiconset/icon120x120.png'
DEBUG [2019-05-24 00:13:29.27]: [34mAdding shields.io image ontop of icon[0m
dyld: Symbol not found: _OBJC_CLASS_$_NSFont
Referenced from: /Library/Frameworks/Mono.framework/Versions/5.12.0/lib/libpangocairo-1.0.0.dylib
Expected in: flat namespace
in /Library/Frameworks/Mono.framework/Versions/5.12.0/lib/libpangocairo-1.0.0.dylib
WARN [2019-05-24 00:13:29.34]: [33m[33mLane Context:[0m
INFO [2019-05-24 00:13:29.34]: {:PLATFORM_NAME=>nil, :LANE_NAME=>"addBadge"}
ERROR [2019-05-24 00:13:29.34]: [31mNo such file or directory # rb_sysopen - 1558656638-staging-green.svg.png[0m
It seems like on the build agent, an svg is being requested instead of a png. Could that have something to do with it?