Plain setup with a couple of S4048-ON switches in VLT setup: SA, SB

From these two switches we have connected a set of LACP port channels towards upstream C6800 (VSS), ToR switches and F5 BIG-IPs.

Now, our issues start when we want to upgrade the S4048-ON pair.
While reloading one VLT peer, we get massive packet drops 50% on the LACP port channels.
The connected LACP devices don't register that the links to the reloading VLT peer are down, and it takes about 1-2 minutes until it converges.

As I am not that familiar with DNOS, how can it be that the S4048 don't shut down interfaces at once while reloading?

As a workaround, we have tried to manually shut down the interfaces on the VLT peer that we want to reload, and that as expected works like a charm with convergence <1s.

interface Port-channel 51
 description Po51 - ny LACP til cisco
 no ip address
 switchport
 no spanning-tree pvst err-dis cause invalid-pvst-bpdu
 lacp fast-switchover
 vlt-peer-lag port-channel 51
 no shutdown
interface TenGigabitEthernet 1/47
 description Po51 - cisco lacp
 no ip address
 mtu 9216
!
 port-channel-protocol LACP
  port-channel 51 mode active


interface TenGigabitEthernet 1/48
 description Po51 - cisco lacp
 no ip address
 mtu 9216
!
 port-channel-protocol LACP
  port-channel 51 mode active

S4048-ON-2-01#sh vlt br
 VLT Domain Brief
------------------
 Domain ID:                      2
 Role:                           Primary
 Role Priority:                  1
 ICL Link Status:                Up
 HeartBeat Status:               Up
 VLT Peer Status:                Up
 Local Unit Id:                  0
 Version:                        6(8)
 Local System MAC address:       f4:8e:38:35:1c:08
 Remote System MAC address:      f4:8e:38:35:1d:08
 Remote system version:          6(8)
 Delay-Restore timer:            4 seconds
 Delay-Restore Abort Threshold:  60 seconds
 Peer-Routing :                  Disabled
 Peer-Routing-Timeout timer:     0 seconds
 Multicast peer-routing timeout: 150 seconds