The upper left key (left of "1") doesn't work on macOS for some layouts, specifically those that use the ISO physical layout. This is a regression caused by this upstream commit:
It was pulled in to ThinLinc in bug 7158 for 4.9.0.
Upstream bug report/pull request:
(In reply to Pierre Ossman from comment #0)
> It was pulled in to ThinLinc in bug 7158 for 4.9.0.
Typo. It was for 4.10.0.