Just launched xcode 1Version 11.0 (11A420a) and built and ran my app. The popup buttons show up as just the title, no box around it and no icon on the right to indicate a dropdown. The popup still works, just the UI is messed up.
Also, NSToolbar items with icons are messed up. Either the icon is empy or a weird image shows up. They work fine in xcode 10.
See release notes for Mac OS 10.15 SDK. Only Catalina uses it.
If you are executing on Mojave, the SDK version being used is the 10.14 SDK.
So this bug seems to only occur on Mac OS SDK version 10.15.
I think you are corrrect.
Xcode 10 does not support building for Mac OS 15 SDK, only Xcode 11 does.
So it is probably a specific Mac OS 15 SDK bug.
Apps built on Xcode 10, will target Mac OS 14 SDK, even when run on Catalina.
Upgrading to Catalina 10.05.1 solves some problems but XCode 11.1 still reports the following spurious errors:
2019-10-30 12:47:27.642240-0600 Appname[5299:71250] Metal API Validation Enabled
2019-10-30 12:47:27.760170-0600 Appname[5299:71250] [Nib Loading] Unknown class NSBoxCustom in Interface Builder file at path /Users/myname/Library/Developer/Xcode/DerivedData/Appname-hkjlvhmirshcnhesgvusrbzzbezi/Build/Products/Debug/Appname.app/Contents/Resources/Base.lproj/MyDocument.nib.
An update:
This weird visual corruption only happens when the app is being debugged from Xcode 11.
It does not happen if you start the app from the Dock.
So I was able to release my app when it was built with Xcode 11.
I have just learned to ignore the visual corruption when debugging the app from Xcode.
XCode 11.5 fixes the bugs like this that I had.
After I cleaned and rebuilt my application these spurious errors reappeared, so no, Apple didn't fix this.