MDEV-15053 refactored the InnoDB buffer pool and page flushing. It unfortunately included one unintended functional change that seriously hurts performance in workloads where the buffer pool is big, but the working set is even bigger. The termination condition for a loop was misplaced:
This bug will cause a permanent drop of throughput when a write-heavy database instance starts thrashing.
MDEV-23399 is suggesting larger changes to data structures and algorithms that could be beneficial in such workloads.