Re-Notification for some services do no work

Hello everybody!
I have a strange phenomenon:
My entire system is configured via Director.
I have 2 Host notification templates, one for e-mail, one for sms and 2 Service notification templates, as well one for e-mail and one for sms.
On all hosts in all groups except for one group, called “Cooling Units” Host- and Servicenotification as well as the re-notification is working like a charm.

This notification service is applied to any service having the variable “notifyservice” set. Which is for all hosts except for cooling units:

apply Notification “Service Notification” to Service {
command = “Service Alarm By Email”
interval = 30m
period = “24/7Mon-Sun”
zone = “dontsay”
assign where service.vars.notifyservice
states = [ Critical, OK, Unknown, Warning ]
types = [ Acknowledgement, Custom, Problem, Recovery ]
user_groups = [ “Servicedesk Group” ]
}

This one applies only to my cooling units.

apply Notification “Temperature Rising Alarm Notification (SMS)” to Service {
command = “Service Alarm By SMS”
interval = 30m
period = “24/7Mon-Sun”
zone = “dontsay”
assign where service.vars.trf == “rising”
states = [ Critical, OK, Unknown, Warning ]
types = [ Acknowledgement, Custom, Problem, Recovery ]
user_groups = [ “Truhenalarm Group” ]
}

Both stanzas are displayed in resolved mode, so the underlying templates are included.

The apply rules work, after rollout in Director I see the corresponding notification groups/users associated to the service of my cooling units. Also I see those which are associated to all other services! The apply rules work as expected.
(Cooling Units and rest of IT infrastructure have different responsible persons).

An IT service reaches a hard state, a notification event is sent, re-notification comes every 30 minutes as that is the default.
A cooling unit warms up above the warning and critical thresholds and reaches after some rechecks a hard state. A notification is sent, but NO re-notification is sent, even though the apply rules are almost identical as you can see.

The difference is the rules’ name and the assign stanza, but I doubt, that this is the issue? And the assign stanza is working and the 1st notification is sent in all cases, the problem is the re-notification…

Anyone who can help?
Thanks, Felix