SUMMARY
I tried running the Flatpak nightly (master) build under Wayland and it would crash after a few minutes of use.

STEPS TO REPRODUCE
1. flatpak run --command=sh --devel org.kde.skrooge ~/Documents/p/finance/pages.sk
2. Enter some transactions ("operations"). There may be a connection with Skrooge's pop-up window containin matching text inputs.

OBSERVED RESULT
Four times in an hour, Skrooge crashed. I folllowed https://docs.flatpak.org/en/latest/debugging.html to get a backtrace:

#0  0x00007ffff6a54ad0 in QCoreApplication::notifyInternal2(QObject*, QEvent*) ()
    at /usr/lib/x86_64-linux-gnu/libQt5Core.so.5
#1  0x00007ffff799253d in QCompleter::eventFilter(QObject*, QEvent*) ()
    at /usr/lib/x86_64-linux-gnu/libQt5Widgets.so.5
#2  0x00007ffff6a548c3 in QCoreApplicationPrivate::sendThroughObjectEventFilters(QObject*, QEvent*) ()
    at /usr/lib/x86_64-linux-gnu/libQt5Core.so.5
#3  0x00007ffff762616e in QApplicationPrivate::notify_helper(QObject*, QEvent*) ()
    at /usr/lib/x86_64-linux-gnu/libQt5Widgets.so.5
#4  0x00007ffff6a54b5a in QCoreApplication::notifyInternal2(QObject*, QEvent*) ()
    at /usr/lib/x86_64-linux-gnu/libQt5Core.so.5
#5  0x00007fffee44609f in  () at /usr/lib/x86_64-linux-gnu/libQt5WaylandClient.so.5
#6  0x00007ffff762a37d in QApplication::setActiveWindow(QWidget*) () at /usr/lib/x86_64-linux-gnu/libQt5Widgets.so.5
#7  0x00007ffff762a44b in QApplicationPrivate::notifyActiveWindowChange(QWindow*) ()
    at /usr/lib/x86_64-linux-gnu/libQt5Widgets.so.5
#8  0x00007ffff6f10965 in QGuiApplicationPrivate::processActivatedEvent(QWindowSystemInterfacePrivate::ActivatedWindowEvent*) () at /usr/lib/x86_64-linux-gnu/libQt5Gui.so.5
#9  0x00007ffff6eea56c in QWindowSystemInterface::sendWindowSystemEvents(QFlags<QEventLoop::ProcessEventsFlag>) ()
    at /usr/lib/x86_64-linux-gnu/libQt5Gui.so.5
#10 0x00007ffff6eea7d8 in QWindowSystemInterface::flushWindowSystemEvents(QFlags<QEventLoop::ProcessEventsFlag>) ()
    at /usr/lib/x86_64-linux-gnu/libQt5Gui.so.5
#11 0x00007ffff6a80b81 in QObject::event(QEvent*) () at /usr/lib/x86_64-linux-gnu/libQt5Core.so.5
#12 0x00007ffff762617f in QApplicationPrivate::notify_helper(QObject*, QEvent*) ()
    at /usr/lib/x86_64-linux-gnu/libQt5Widgets.so.5
#13 0x00007ffff6a54b5a in QCoreApplication::notifyInternal2(QObject*, QEvent*) ()
    at /usr/lib/x86_64-linux-gnu/libQt5Core.so.5
#14 0x00007ffff6a57591 in QCoreApplicationPrivate::sendPostedEvents(QObject*, int, QThreadData*) ()
    at /usr/lib/x86_64-linux-gnu/libQt5Core.so.5
#15 0x00007ffff6aac8b3 in  () at /usr/lib/x86_64-linux-gnu/libQt5Core.so.5
#16 0x00007ffff256d85b in g_main_context_dispatch () at /usr/lib/x86_64-linux-gnu/libglib-2.0.so.0
#17 0x00007ffff256db08 in g_main_context_iterate.constprop () at /usr/lib/x86_64-linux-gnu/libglib-2.0.so.0
#18 0x00007ffff256dbd3 in g_main_context_iteration () at /usr/lib/x86_64-linux-gnu/libglib-2.0.so.0
#19 0x00007ffff6aabf3f in QEventDispatcherGlib::processEvents(QFlags<QEventLoop::ProcessEventsFlag>) ()
    at /usr/lib/x86_64-linux-gnu/libQt5Core.so.5
#20 0x00007ffff6a5350b in QEventLoop::exec(QFlags<QEventLoop::ProcessEventsFlag>) ()
    at /usr/lib/x86_64-linux-gnu/libQt5Core.so.5
#21 0x00007ffff6a5b790 in QCoreApplication::exec() () at /usr/lib/x86_64-linux-gnu/libQt5Core.so.5
#22 0x000055555555a12d in  ()
#23 0x00007ffff63ec062 in __libc_start_main () at /usr/lib/x86_64-linux-gnu/libc.so.6
#24 0x000055555555a19e in  ()


EXPECTED RESULT
No crashes

SOFTWARE/OS VERSIONS
Linux/KDE Plasma: 
(available in About System) <-- this isn't so relevant for a Flatpak.
Flatpak runtime: org.kde.Sdk/x86_64/5.15

ADDITIONAL INFORMATION
This may be related to the Wayland hangs and crashes I reported in bug 446352