There is an updated pull request by Hoshpak against master on the void-packages repository

https://github.com/Hoshpak/void-packages mono-6.4.0.198
https://github.com/void-linux/void-packages/pull/14696

[WIP] mono: update to 6.4.0.198.
None

A patch file from https://github.com/void-linux/void-packages/pull/14696.patch is attached