That seems to have fixed it!
Full log is here:
Interesting bit in the log is:
2020/06/02 20:03:29 DEBUG : <<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<
2020/06/02 20:03:29 DEBUG : : changeNotify: relativePath="sub1/subsub1/emptyfile1", type=3
2020/06/02 20:03:29 DEBUG : : invalidateDir: absPath="sub1/subsub1"
2020/06/02 20:03:29 DEBUG : sub1/subsub1: invalidateDir: checking dir 2020-06-02 20:03:09.2840688 +0100 BST m=+11.452373501 is zero false
2020/06/02 20:03:29 DEBUG : sub1/subsub1: invalidating directory cache
2020/06/02 20:03:29 DEBUG : : >invalidateDir:
2020/06/02 20:03:29 DEBUG : : >changeNotify:
2020/06/02 20:03:33 DEBUG : /: Getattr: fh=0xFFFFFFFFFFFFFFFF
I guess this fix will find it's way into the next beta.
Many thanks for fixing it!