Originally posted by Jaggid Edje:

As of just now I have finished clearing that first mine, and I did not get any more cores, so 18 was the final total from it.

I also was perplexed because there were new seekers in rooms I had already cleared when I went in. I thought dungeons didn't respawn.
Turns out I had only a single room left unexplored since the last visit, but the total number of seekers I found on this return trip was significantly more than would have fit in the room, and they spread throughout the mine in already-cleared areas.

Respawns in a mine are a new thing. It goes along with respawning royal jelly. I don't think it happens instantly, but probably did it since it took you awhile to return.