Caveat: This looks a lot like the following bug which is marked fixed: https://bugs.kde.org/show_bug.cgi?id=348043 But I'm running 5.27 in 2025 and that bug report is attached to version 5.3 and dated to the year 2015, which makes me believe this is a different problem (although perhaps with the same result). Two apps I use -- Firefox and Telegram -- have exhibited the same problematic behavior: When I maximize them (Super-Pageup) they are drawn so large that their bottom portion is obscured by the Panel (if Panel is the right term for the skinny bar with app icons at the bottom of the screen that's visible outside of an app's full-screen mode).
I'm afraid Plasma 5.27 is unfortunately no longer eligible for support or maintenance from KDE. Plasma is a fast-moving project, and bugs in one version are often fixed in the next one. Please update to Plasma 6 as soon as your distro offers it to you. If you need support for Plasma 5.27, please contact your distro, who bears the responsibility of providing support for older Plasma releases. If this issue is still reproducible in a Wayland session in Plasma 6.3, feel free to re-open this bug report. Thanks for understanding!