I can reply to my own comment, but the speech bubble below yours is greyed out and I cannot. What gives?

Some call it Voodoo, others call it JavaScript, owl in this case to be more precise. None the less, I feel you on that one... Source is, well, source code; and verification in localhost as well as on odoo.sh. The /web/login route is defined here:
https://github.com/odoo/odoo/blob/18.0/addons/web/controllers/home.py#L97 and 'cids' is commonly used throughout those voodoo-files to refer to company_ids. Now, I should mention, that a test in demo.odoo.com did in fact not succeed - which can be due to 18.3 working just that bit differently, or, what I would assume more, the cookies stored on that session are messing with the known default company. In any case, you may actually want to try to clear your browser cookies unless you're operating incognito already. What version of Odoo are you running exactly?
Also, to verify your redirect aren't completely broken for what ever reason, you can try to test a redirect to a portal page, such as /my upon login as well.
And, about commenting: https://www.odoo.com/forum/1/faq/karma - I would assume 1k is the threshold but I have honestly no clue.