ok. sorry to be unclear, it is rather puzzling.
I'll tell you what the situation is:
The fortress is several years old, no features have been found yet (except for the volcano), but we have a dungeonmaster.
Lately we have missed some caravans due to unwise fortification planning.
Several large projects are underway, an outdoors arena, a large vaulted underground cathedral and a second well.
The arena construction orders were halted as should be when I ordered all inside for a siege and could be reset to resume building.
The cathedral however got terribly delayed, as my smoothing designations were ignored, miners just hanging around, going off to dig only freshly designated areas.
undesignating and redesignating finally got the ceilings smoothed.
the well was completed years after designation similarly.
I'm not entirely sure it is the sieges that 'pause' the designations, I may have been confusing the halted arena construction with this designation problem.
I have been training my miners a bit too and they have been recruited during a siege at some point. hat could be a factor, but only to the tile they were planning to mine at that time, not the entire project.
reproducing the effect...I could undesignate everything, then designate a single tile, use the stayindide and watch the jobs.
or recruit the miners while they are going for a job, then release them.