TortoiseGitMerge 'Use this text block' not working

What steps will reproduce the problem?

1. Edit a file in some repo - any change will do
2. Go to Git Commit
3. Right-click the modified file and click Compare with base - TortoiseGitMerge should open
4. Right-click a modified line on the left side and click Use this text block

What is the expected output? What do you see instead?

Expected the line on the right to be replaced by the line on the left.

In the latest release (2.10.0.1) this appears to have no effect; nothing happens to the line on the right. However, TortoiseGitMerge shows the file modified after clicking Use this text block.

What version of TortoiseGit and Git are you using? On what operating system?

TortoiseGit 2.10.0.1 64-bit

git version 2.25.1.windows.1

Windows 10 (version 1909)

Please provide any additional information below.