SUMMARY
Plymouth themes don't apply on startup, but do apply on shut down
KDE neon's default plymouth theme is also unavailable in system settings, although always is used on startup, even when a specific plymouth theme is selected

STEPS TO REPRODUCE
1. Open System Settings > Appearance > Boot Splash Screen
2. Select A Theme
3. Reboot

OBSERVED RESULT
Selected Plymouth theme shows up on shutdown, but not startup

EXPECTED RESULT
Selected Plymouth theme should show up on startup (instead of the unselectable neon plymouth theme) and shutdown

SOFTWARE/OS VERSIONS
macOS: 12.2.1 (HOST)
VMware Fusion: Player 12.2.1
Linux/KDE Plasma: KDE neon 5.24 User Edition (VM)
KDE Plasma Version: 5.24.2
KDE Frameworks Version: 5.91.0
Qt Version: 5.15.3

ADDITIONAL INFORMATION
1.While this is on a VM that I use regularly, I doubt it is VM only
2. I've had plymouth themes actually show on startup and work correct on a fresh install (I used Breeze (Text Mode) for a while.), but after a couple weeks worth of update and switching back to default Breeze, Plymouth on startup only shows the default Neon plymouth theme.
3. Neon's default plymouth theme (aka the one that shows up instead of the custom plymouth theme on startup) is not a select-able option in Boot Splash Screen.
4. Applying a plymouth theme also takes an unusual amount of time, usually around a minute

Pumping System Settings through terminal and posting the output of selecting a plymouth theme.
rad@DripKonq:~$ systemsettings
QQmlEngine::setContextForObject(): Object already has a QQmlContext
file:///usr/share/kpackage/kcms/kcm_landingpage/contents/ui/main.qml:130:9: QML FormLayout (parent or ancestor of QQuickLayoutAttached): Binding loop detected for property "preferredHeight"
file:///usr/share/kpackage/kcms/kcm_landingpage/contents/ui/main.qml:20:9: QML FormLayout (parent or ancestor of QQuickLayoutAttached): Binding loop detected for property "preferredHeight"
QQmlEngine::setContextForObject(): Object already has a QQmlContext
file:///usr/lib/x86_64-linux-gnu/qt5/qml/QtQuick/Controls.2/org.kde.desktop/ScrollView.qml:85:25: QML ScrollBar: Binding loop detected for property "visible"
file:///usr/lib/x86_64-linux-gnu/qt5/qml/QtQuick/Controls.2/org.kde.desktop/ScrollView.qml:85:25: QML ScrollBar: Binding loop detected for property "visible"
qml: The item SubCategoryPage_QMLTYPE_114(0x560a2bc0ca70) is already in the PageRow
QQmlEngine::setContextForObject(): Object already has a QQmlContext
file:///usr/share/kpackage/kcms/kcm_plymouth/contents/ui/main.qml:79:5: QML Connections: Implicitly defined onFoo properties in Connections are deprecated. Use this syntax instead: function onFoo(<arguments>) { ... }
"Could not convert argument 0 at"
         "onChangedEntriesChanged@file:///usr/share/kpackage/kcms/kcm_plymouth/contents/ui/main.qml:75"
"Passing incompatible arguments to C++ functions from JavaScript is dangerous and deprecated."
"This will throw a JavaScript TypeError in future releases of Qt!"
file:///usr/lib/x86_64-linux-gnu/qt5/qml/QtQuick/Controls.2/org.kde.desktop/ScrollView.qml:85:25: QML ScrollBar: Binding loop detected for property "visible"
file:///usr/lib/x86_64-linux-gnu/qt5/qml/QtQuick/Controls.2/org.kde.desktop/ScrollView.qml:85:25: QML ScrollBar: Binding loop detected for property "visible"
qml: The item SubCategoryPage_QMLTYPE_114(0x560a2bc0ca70) is already in the PageRow
"Could not convert argument 0 at"
         "onChangedEntriesChanged@file:///usr/share/kpackage/kcms/kcm_plymouth/contents/ui/main.qml:75"
"Passing incompatible arguments to C++ functions from JavaScript is dangerous and deprecated."
"This will throw a JavaScript TypeError in future releases of Qt!"
"Could not convert argument 0 at"
         "onChangedEntriesChanged@file:///usr/share/kpackage/kcms/kcm_plymouth/contents/ui/main.qml:75"
"Passing incompatible arguments to C++ functions from JavaScript is dangerous and deprecated."
"This will throw a JavaScript TypeError in future releases of Qt!"
Debug message from helper: Running update-alternatives --list default.plymouth now
Debug message from helper: Running update-alternatives --set  now
Debug message from helper: Running update-initramfs -u  now
kf.auth: "Did not receive a reply. Possible causes include: the remote application did not send a reply, the message bus security policy blocked the reply, the reply timeout expired, or the network connection was broken."
Warning from helper: QProcess: Destroyed while process ("/usr/sbin/update-initramfs") is still running.
rad@DripKonq:~$