Some more things I’ve tried, all with the same core issue happening again and again:
- Factory reset all Bluesound devices, not touching any custom settings afterwards
- Remove any fixed IP & network name customization from the speakers (in router interface)
- Disabling and re-enabling the devices in Roon
- Grouping all four Bluesound devices with BluOS, playing back via Roon
- Grouping all four BS devices with Airplay 2 and playing back via Tidal — no issues at all
- Restarting my Router, restarting all the devices in the network
At this point I’m returning them to the store while I can. Quite bummed about this, as the feature combination and sound where ideal for secondary/tertiary listening rooms. I’m probably going to use RPi+HAT+Ropieee installations with some “non-smart” (dumb? 
) speakers now…
I’d be interested to hear what QA says about this issue, but unfortunately I cannot really wait. With all the different things I’ve tried, I can hardly imagine that this is an exclusive issue.
EDIT:
I’ve returned the speakers to the store for full credit and bought some passive speakers that I plan to run with a RPi+HifiBerry-AMP2+Ropieee config. Thanks HiFiKlubben for an easy return!