I was aware of the flags, just not whether there is a difference in the emphasises of tell and notify; whether both amount to the same - announcing the the availability of both or either?
It seems that option dhcpv6_assignall '0' is causing the issue
Assign all viable DHCPv6 addresses in statefull mode; if disabled only the DHCPv6 address having the longest preferred lifetime is assigned
The question thus is whether the lifetime of the ULA is competing with the lifetime for GUA despite each covering a different address space?
Since the preferred lifetime is being set per interface and not per address space the set value applies to the interface where both, ULA and GUA, are present.
Should there not be a distinction in considering the address space, ULA <-> GUA, if the longest preferred lifetime is the deciding factor for this setting, else it would seem to be a race condition.