Adding NFS storage in PVE 5.2 causes OOM on NFS server

[ppo]

I have 2 servers: Proxmox and NFS/Samba. They are directly connected by cable.
My PVE server has been upgraded from old PVE 2.x version to the newest one 5.2.
Upgrade has been performed from scratch by installing to new hdd from usb-drive with pve.iso.

Spoiler: pveversion

pveversion -v
proxmox-ve: 5.2-2 (running kernel: 4.15.17-1-pve)
pve-manager: 5.2-1 (running version: 5.2-1/0fcd7879)
pve-kernel-4.15: 5.2-1
pve-kernel-4.15.17-1-pve: 4.15.17-9
corosync: 2.4.2-pve5
criu: 2.11.1-1~bpo90
glusterfs-client: 3.8.8-1
ksm-control-daemon: 1.2-2
libjs-extjs: 6.0.1-2
libpve-access-control: 5.0-8
libpve-apiclient-perl: 2.0-4
libpve-common-perl: 5.0-31
libpve-guest-common-perl: 2.0-16
libpve-http-server-perl: 2.0-8
libpve-storage-perl: 5.0-23
libqb0: 1.0.1-1
lvm2: 2.02.168-pve6
lxc-pve: 3.0.0-3
lxcfs: 3.0.0-1
novnc-pve: 0.6-4
proxmox-widget-toolkit: 1.0-18
pve-cluster: 5.0-27
pve-container: 2.0-23
pve-docs: 5.2-3
pve-firewall: 3.0-8
pve-firmware: 2.0-4
pve-ha-manager: 2.0-5
pve-i18n: 1.0-5
pve-libspice-server1: 0.12.8-3
pve-qemu-kvm: 2.11.1-5
pve-xtermjs: 1.0-5
qemu-server: 5.0-26
smartmontools: 6.5+svn4324-1
spiceterm: 3.0-5
vncterm: 1.5-3
zfsutils-linux: 0.7.8-pve1~bpo9

Old kvm and openvz images were successfully migrated to new system.
NFS storage has been mounted to new server for backup purpose.
After a few weeks I've got denial of Samba service on server which is also acts as NFS server.
After getting access to NFS/Samba server I noticed high memory usage by mountd service.
I restarted NFS and then Samba daemons there and forgot about issue for a few weeks.
But denial of service on NFS/Samba server has been occurred again.
I've executed this one liner on NFS server and memory stopped to increase after I removed NFS share in Proxmox GUI:

Spoiler: mountd memory consumption

while :; do ps | grep mountd | grep -v grep | awk '{print $3}'; sleep 3; done
6260
6260
6264
6264
6264
6268
6268
6268
6276
6276
6276
6280
6280
6280
6280
6284
6284
6284
6292
..
..skipped..
..
6492
6492
6492
6496
6496
6496
..
..at this point I removed NFS share in PVE GUI
..
6504
6504
6504
6504
6504
6504
6504
..
..no more increase for long time
..

I've never encountered this with old PVE 2.x.
I've similar setup with PVE 3.x and NFS/Samba server and also never got such issue.