I'm seeing the same crash in iOS 14.4.2, 14.4, and 14.3 in an iPad app. First started seeing them on June 8.
NSInvalidArgumentException: -[__NSCFNumber compare:]: nil argument
0 CoreFoundation ___exceptionPreprocess
1 libobjc.A.dylib _objc_exception_throw
2 CoreFoundation __CFThrowFormattedException
3 CoreFoundation -[__NSCFNumber compare:].cold.1
4 CoreFoundation -[__NSCFNumber compare:]
5 CoreFoundation -[__NSCFNumber isEqualToNumber:]
6 CoreText +[FontAssetDownloadManager filterIncompatibleAsset:]
7 CoreText +[FontAssetDownloadManager availableMobileAssetsWithOptions:error:]
8 CoreText +[FontAssetDownloadManager availableMobileAssetsWithOptions:manager:error:]
9 CoreText -[FontAssetDownloadManager mobileAssetsForUnmatched:]
10 CoreText -[FontAssetDownloadManager downloadFontAssets]
11 CoreText ___CTFontDescriptorMatchFontDescriptorsWithProgressHandler_block_invoke
12 libdispatch.dylib __dispatch_call_block_and_release
13 libdispatch.dylib __dispatch_client_callout
14 libdispatch.dylib __dispatch_lane_serial_drain$VARIANT$mp
15 libdispatch.dylib __dispatch_lane_invoke$VARIANT$mp
16 libdispatch.dylib __dispatch_workloop_worker_thread
17 libsystem_pthread.dylib __pthread_wqthread
18 libsystem_pthread.dylib _start_wqthread