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I can reproduce this on x86_64 Fedora 22.  It is upstream Mono bug https://bugzilla.xamarin.com/show_bug.cgi?id=8935.

Just upgraded to f24 (from f22), and in research why my keepassx icon disappeared, ended up here.  So, can confirm that this bug is still active in the current release.

I have found now a patch that resolves this issue.
It is mentioned here: https://bugzilla.xamarin.com/show_bug.cgi?id=14976
I will update the mono packages accordingly over the next days.

Fantastic, thanks!  Thinking ahead to testing out your fix, though: despite being able to run xkeepass happily (in all respects except for that icon), there's not a mono package to be found on my system*.  Or, are there no runtime components but only build-time?  If that's the case, I'll pull the xkeepass SRPM and load up on build reqs for testing.

*"rpm -qa | grep -i mono" returns a few monospaced font files, and that's it :)

mono-4.2.4-2.fc24 has been submitted as an update to Fedora 24. https://bodhi.fedoraproject.org/updates/FEDORA-2016-dbe5ffa1cc

@Alec: I think you are confusing two packages: keepassx and keepass. 

https://admin.fedoraproject.org/pkgdb/package/rpms/keepassx/ is not using Mono at all: http://pkgs.fedoraproject.org/cgit/rpms/keepassx.git/tree/keepassx.spec
It is this software: http://www.keepassx.org/

We are talking here about https://admin.fedoraproject.org/pkgdb/package/rpms/keepass/. It is this software: http://keepass.info/
It seems in http://pkgs.fedoraproject.org/cgit/rpms/keepass.git/tree/keepass.spec there is no direct dependancy on Mono for installation. But during the build the dependancy for mono(System) = 4.0.0.0 is picked up automatically. So "dnf install keepass" will always check that Mono is installed as well.

To test keepass with the updated Mono packages, which will go into the updates testing repository within this day, see https://fedoraproject.org/wiki/QA:Updates_Testing

ahh, that's it.  keepass that uses mono is the newer version, compatible with the windows program but not the android app I need: utilizing the older DB format.  Thus I am using keepassx (specifically, keepassx0), which does not depend upon mono: so it must have a blank icon for other reasons.  Sorry for the confusion: but surely the mono-using people are happy for the bugfix :)

In fact, a more alert user put in bugzilla 1349348 yesterday for keepassx0, and a patch has already been pushed, and it works.

Hooray for opensource software and a great community!

mono-4.2.4-2.fc24 has been pushed to the Fedora 24 testing repository. If problems still persist, please make note of it in this bug report.
See https://fedoraproject.org/wiki/QA:Updates_Testing for
instructions on how to install test updates.
You can provide feedback for this update here: https://bodhi.fedoraproject.org/updates/FEDORA-2016-dbe5ffa1cc

https://bodhi.fedoraproject.org/users/bollocks writes on Bodhi:

Keepass and Pinta are working good for me and both use Mono. I didn't even know KeePass was supposed to have a tray icon. Something weird seems to be happening. I'm running i3 with i3bar and I see this: http://i.imgur.com/RhDxyaV.png
Clicking on the icon brings up KeePass but usually tray icons end up on the right side like the hexchat icon in the bottom right.

Is this a new bug? What should we do?

mono-4.2.4-2.fc24 has been pushed to the Fedora 24 stable repository. If problems still persist, please make note of it in this bug report.