Currently, run-web-platform-tests is hardcoded to work with a release build. I will submit a PR adding --debug and --release args similar to other build/run scripts.
Pull request: https://github.com/WebKit/WebKit/pull/19869
Committed 270113@main (8dedd163435d): <https://commits.webkit.org/270113@main> Reviewed commits have been landed. Closing PR #19869 and removing active labels.
<rdar://problem/117860249>