I'm not sure I've got the details exactly right here but the best of my understanding is: On linux64 debug builds, try uses the flag "-u mochitest-chrome", as opposed to "-u mochitest-o" on other platforms (perhaps related to taskcluster migration?). But mach try doesn't appear to know anything about mochitest-chrome: running "mach try -b d -p linux64 path/with/chrome-mochitests" produces a try command with mochitest-o. A resulting try job then runs a bunch of non-chrome tests and doesn't actually run the chrome mochitests. Very frustrating. The discrepancy between different platforms may well be unfixable (e.g., no valid try syntax accurately captures "mach try -b d -plinux64,win64 path/with/chrome-mochitests") but for the time being, making a try run that includes chrome mochitests on linux64 debug produce a fatal error would remove the easy pitfall.
:armenzg -- Can you clarify the history here? I'm wondering why mochitest-oth was split into mochitest-a11y and mochitest-chrome for taskcluster, and if that's the "right" approach for mochitest-other on other platforms. (I blindly copied the same mochitest-a11y/chrome task definitions for linux64-opt and linux64-pgo, but am second-guessing that in view of this bug.)
I don't remember the exact reason of the split. jmaher and I workerd on it. IIRC it was to make the jobs run faster. Just to be on the same page, here's a summary: I see the mochitest-chrome split for Linux 64 (opt,debug,pgo) and Linux (debug only)  I see mochitest-other running for all platforms except Linux64 debug (running on TaskCluster)  Should we split mochitest-other in here? I believe we can temporarily adjust the try syntax parser for the TC jobs  to include mochitest-chrome when mochitest-o is specified. That way we won't have to split all the jobs in here.  https://treeherder.mozilla.org/#/jobs?repo=mozilla-central&revision=3c5025f98e561a20e24d97c91a9e4e0ec28015ea&filter-searchStr=mochitest-chrome&group_state=expanded  https://treeherder.mozilla.org/#/jobs?repo=mozilla-central&revision=3c5025f98e561a20e24d97c91a9e4e0ec28015ea&filter-searchStr=mochitest-other&group_state=expanded  https://dxr.mozilla.org/mozilla-central/source/taskcluster/taskgraph/try_option_syntax.py#69
(In reply to Armen Zambrano [:armenzg] - Engineering productivity from comment #2) Thanks Armen. > I believe we can temporarily adjust the try syntax parser for the TC jobs >  to include mochitest-chrome when mochitest-o is specified. > That way we won't have to split all the jobs in here. That sounds like a good idea to me.