You need to turn off case sensitivity: change "gitdir:" to "gitdir/i:"
[includeIf "gitdir/i:C:/Work/"]
path = .gitconfig-work
[includeIf "gitdir/i:C:/My/Dev/"]
path = .gitconfig-my
from:
https://github.com/Microsoft/vscode/issues/62921#issuecomment-437693020