My "lastshare" has been stuck at 15445xxxxx for WEEKS now!!!

Any explanation for this?

Yes: Nobody got a better share in that time, thats all.
Nothing to worry about.

Mine on

What is the reason behind this?
How were the shares higher before difficulty drop and now it's stuck?

same reason you can toss a pair of dice 1000 times and not toss a pair of 6's for a total of 12.

Random luck

Random luck i get..
But tens of miners having the same exact lastshare +- 500 is not random at all lol

lol so is mine but this is really just random.