After some digging around, it turns out that gtm.js relies on their `dataLayer.push` method returning `true`, otherwise it prevents the default action when tracked links are clicked. I'm very surprised that such breakage hasn't been more widely reported, but the fix is thankfully trivial. I'll attach a patch here ASAP.
Bug 1713687 Comment 5 Edit History
Note: The actual edited comment in the bug view page will always show the original commenter’s name and original timestamp.
(commented on the wrong bug)