Context :

1. we have a multibranch job, configured for discovering branches / MRs / ..., with such pipeline

   pipeline {
       agent { label ... }
   
       options {
           ...
           throttleJobProperty(
                   categories: ['myCategory'],
                   throttleEnabled: true,
                   throttleOption: 'category'
           )
       }
   
       ...
   }
   

2. "myCategory" is configured properly
3. but also configured for "Suppress automatic SCM triggering" (from Branch API plugin)
4. so we have such jobs, where we manually triggered some job, under some "customer driven" needs

Fact :
Throttling works fine with respect to configuration / category, for jobs that have been scheduled at least once, but throttling seems to not be honored for the first run of every job in this multibranch job.