I managed to replicate this and I used git bisect to find the troublesome commit.
This is good news because there is an easy work-around --attr-timeout 1s (or choose 60s if your stuff doesn’t change much).
Even --attr-timeout 1ns fixes the problem.
I have no idea why this affects samba though!
commit fc32fee4add93e7a3bda180c7c1088f93c759d13
Author: Nick Craig-Wood <nick@craig-wood.com>
Date: Fri Mar 2 16:39:42 2018 +0000
mount, cmount: add --attr-timeout to control attribute caching in kernel
This flag allows the attribute caching in the kernel to be controlled.
The default is 0s - no caching - which is recommended for filesystems
which can change outside the control of the kernel.
Previously this was at the default meaning it was 60s for mount and 1s
for cmount. This showed strange effects when files changed on the
remote not via the kernel. For instance Caddy would serve corrupted
files for a while when serving from an rclone mount when a file
changed on the remote.