• create matrix projectA
• add axis named x, values: a b
• add build step that touches a file file.txt
• archive file.txt as artifact, and fingerprint it
• select to aggregate downstream test results

• duplicate projectA, name it projectB
• add build step to copy artifact file.txt from projectA/x=$x
• fingerprint file.txt
• seletc to publish testing tools report, do not fail build if tests not updated
• build projectB once to create workspace, put a testresult xml in each

• add post-build step to projectA 'build other projects' and select projectB
• build projectA

Test results from B do no make it into A:

• status page of projectA, configuration a shows 'Latest Test Result (no tests)'
• latest build page of projectA, configuration a shows 'Aggregated Test Result (no tests)'

Following the same procedure, but for a normal project gives all the exepcted results though.

(attached a zip with project config/workspaces that should be ready to try)