Coverage chart / Pixelmator Pro
Floodtide does not update Pixelmator Pro
Pixelmator Pro comes from the Mac App Store, which macOS already updates. Rather than leave you to find that out after paying, here is the whole reason.
Why not
App Store titles are updated by the system, on Apple's schedule, with Apple's receipts. Floodtide has no way to reach into that and no reason to: the job is already done, and duplicating it would mean two things fighting over the same bundle. Floodtide covers the apps macOS does not look after, which is most of what a working Mac actually runs.
Turn on automatic updates in App Store settings and Pixelmator Pro takes care of itself.
Floodtide also reads the Sparkle feed of any installed app that publishes one, which is a separate path from the Homebrew list. If Pixelmator Pro ships a Sparkle feed on your Mac, it will appear in your update list even though it is not on the coverage chart.
What this says about the rest
MacUpdater kept a hand-maintained database of every Mac app, which is why it could claim near total coverage right up until the day it could not maintain it any more. Floodtide reads sources the vendors already keep current, so the coverage list is smaller and it does not go stale. The trade is deliberate: a shorter list you can trust beats a longer one you cannot check.
Questions about Pixelmator Pro
Does Floodtide update Pixelmator Pro?
No. Pixelmator Pro comes from the Mac App Store, which macOS already updates. App Store titles are updated by the system, on Apple's schedule, with Apple's receipts. Floodtide has no way to reach into that and no reason to: the job is already done, and duplicating it would mean two things fighting over the same bundle. Floodtide covers the apps macOS does not look after, which is most of what a working Mac actually runs.
Will Floodtide ever cover Pixelmator Pro?
Only if Pixelmator Pro starts shipping outside the App Store. While macOS updates it, Floodtide covering it as well would mean two updaters writing to the same bundle.
Does Floodtide hide apps it cannot update?
No. Floodtide shows an app it cannot handle with the reason attached, in the app and here. An updater that quietly omits what it cannot do is one you cannot use to answer the question "is this Mac up to date", which is the only question that matters.