Seeking help debugging check_period processing

One of our checks is seemingly not being limited by the check_period directive.
Just hoping for some guidance on where to look for clues as to why this is.

We’ve got a “9to5” definition that uses what i assume are UTC times adjusted for the preferred timezone for the check_period and listing only mon-fri (no weekends).

If Icinga doesn’t understand our value for 9to5, does Icinga silently ignore it? If it does recognize the definition, is there some indication somewhere that the check is actually suspended for the appropriate time?

thnx in advance and apologies if this is not the correct forum