In April, a memory corruption bug in the Onlyoffice Documents app was reported to the Onlyoffice development team. The bug was caught by GrapheneOS' hardened_malloc memory tagging extension (MTE) implementation, and occurred every time a user attempted to create or open a document. The issue was fixed by the Onlyoffice developers in version 8.2.3 of the app, released two days ago.
The bug was also reproducible with stock PixelOS' usage of MTE with Android's standard memory allocator, but PixelOS did not produce a stacktrace for this specific crash. It is worth noting that GrapheneOS forces usage of memory tagging for apps that use custom crash handlers, which could be why GrapheneOS was able to easily provide a tombstone while PixelOS did not.
GrapheneOS provides a way to run an app with the stock PixelOS' implementation of MTE. This can be done by disabling Hardened memory allocator (hardened_malloc) for the app while keeping memory tagging enabled for it. When Onlyoffice Documents was run this way, MTE marked the bug as a buffer overflow.
GrapheneOS makes it easy for users to save a tombstone from a crash without needing to gather a bugreport via Developer options: when memory tagging on GrapheneOS crashes an app, the user can select "More info" in the crash notification, and then select "More info" again in the screen that appears, which will display the tombstone with the option to save it. It can then be forwarded to the app developers for debugging purposes.