When running MC24 on the Mac have you tried it without using the BubbleUPnP Openhome layer on the Mac? I'm wondering if something has changed in MC24 that makes it not work properly with it.
Also were you running a current build of MC24 on the Mac? There were a few builds that had issues with Bubble.
I'd still like to see if I can duplicate your issue but I need to get as close as possible to your setup.
We've got a QNAP NAS that we can use for the server side and BubbleUPnP control point on an android tablet and of course the Mac Mini with all version of MC running on it.
Making some guesses as to your exact configuration, I tried this and it worked flawlessly with the latest MC24 for the Mac (24.0.45).
Latest BubbleUPnP on an android tablet
QNAP NAS TS-251 4.3.4 with the default DLNA server running
Mac Mini running High Sierra and MC
24.0.45.
Latest OpenHome server software on the Mac Mini
I tested this first with the created OH Renderer and then with MC's built in renderer.
With the OH renderer for MC on the Mac set to use gapless playback it worked perfectly and transitioned tracks gaplessly
Alternately, using MC's builtin renderer and with the BubbleUPnP control point on the android tablet set to NOT support gapless, everything worked fine except for gapless playback. With gapless set for MC's renderer on the BubbleUPnP android app settings, gapless playback worked perfectly on the Mac Mini MC.
So I'm unable to reproduce what you are seeing.
One thing I would check, when I first tried this, my firewall was disabled on the Mac. So I turned it on. I went into the
System Preferences->Security & Privacy->Firewall->Firewall Options
and Media Center 24 wasn't there. I added it.