Sync problem when using external effects

Workstation setup:

OS: Windows 10 Pro 64-bit
Sound cards: 1) RME HDSPe AES, 2) RME HDSPe MADI (both clocked externally off a Lavry Blue)
Host: Cubase Pro 10.5.20


Using external effects forces Cubase intermittently out of sync with the sound card. I need to keep resetting latency buffer values in the sound card for the sync to resume (sometimes it does, sometimes it doesn’t). Going out of sync can be gradual (the sound starts to stutter, crackle before it stops completely), or abrupt resulting in silence: Cubase indicators still showing the sound as being played while the sound card receiving no signal).

This seems to happen only when external effects are used. Otherwise, the system is stable.

Loading/offloading or dragging an external effect from one track/channel to another seems to increase the chances of loosing sync.

(Not sure if connected to the issue: dragging an active external effect from one channel to another changes its state to bypassed.)

Has anyone experienced the sync issue as described above?