using auto-merge in git won't add file automatically

I use Meld (3.20.2) with git to solve merge/rebase conflicts. Here is my .gitconfig :

[merge]
	tool = meld
[mergetool]
	keepBackup = false
[mergetool "meld"]
	path = C:/Program Files (x86)/Meld/Meld.exe
	useAutoMerge = true

When a file conflicts and is able to auto-merge, the file is not automatically added to files to be commited. Also a rebase will not continue automatically. So I run mergetool to solve the potential unresolved "conflict" and get :

$ git mergetool
No files need merging

I expect the automatically merged files to be added to commited files like when there is no conflict.

I also tried setting mergetool.meld.trustExitCode to true or false but his has no effect.