From a discussion elsewhere:

The subbuild module attempts to add those links to step's presentation¹, and the presentation object used here is a nested step:
https://flutter.googlesource.com/recipes/+/refs/heads/main/recipes/engine_v2/engine_v2.py#111

I don't know if you can add links to a nested step, but seems like it doesn't get displayed (that could be a bug in Luci). You could pass in the presentation of an empty step and attach the links to that step instead as a workaround. Or change your subbuild module to add the links to the schedule step. If the schedule step ends up being the only step there, then maybe the nesting isn't needed at all? You could even change the step_name of schedule to launch builds and add the links there directly. You may even be able to remove the id based links.

P.S.: what's up with the weird get buildbucket id steps in the screenshot? Those seem to not be doing anything.