So users have been reporting data corruption when using Bcache and a combination of Linux 5.0 + GCC 9.
Multiple users are confirming “massive filesystem corruption” including by the stock Fedora 30 kernel which is built by GCC9 and based on Linux 5.0. There is also this related Red Hat bug entry.
https://www.phoronix.com/scan.php?page=news_item&px=BCache-Corruption-Linux-5
Bugs have been reported:
https://bugzilla.kernel.org/show_bug.cgi?id=203573
and
https://bugzilla.redhat.com/show_bug.cgi?id=1708315
The issue seems to be in corruption of metadata on the frontend so if users disable the frontend they should be safe until a fix is released although users are being asked to not upgrade until a patch is out.