HMM I dont know how long it took for some Stalhrim deposits to respawn. Yet had the same thing happen the Stalhrim showed back up, but I could not mine it. All I did was leave the area and went back to Skyrim. A few weeks later I went back to the Island and then could then mine the Stalhrim again with no problems.
Did you wait 31 days or some thing less than that? Needs 31 in game days for the deposits to have a chance to respawn, and even then, they may not respawn for more time after that. Note, they usually respawn in 31 days.
From official sorces it says this Stalhrim deposit will respawn in 10 days. Now it did say that if you stay in the cell that the deposits are in then they will not be mineable. Since the counter for the respawn deposit will reset (Start all over). Dont know if that is the issue since he stated he left the area, but did he leave far enough for the counter to finish or did he go back every few days to check. If so then everytime he went back to check the counter reset.
Deposits typically replenish after 30-35 days, much like other ores.
However if there is an object behind the deposit that can be interacted with, such as the body of a Draugr
there will be no option to mine the deposit, only to activate the object.
Other locations you can find Stalhrim are:
Benkongerike Great Hall - Bloodskal Barrow - Gyldenhul Barrow,- Kolbjorn Barrow - Raven Rock Mine - Unmarked glacial island, northeast of Northshore Landing.
(If the Stalhrim has replenished and it can't be accessed, simply leave the area and come back again, this solves the mining bug.) - should normally.
went back every few days and ended up waiting a total of 30 days.=Counter rest everytime you went back the Stalhrim deposit counter respawn reset everytime you went back. Yes the Stalhrim deposit is showing, but that might be the reason you cant mine it.
Try this: Save your game then go to Skaal Village wait for 10 days, go back on the 11 day and see if you can mine it then. If not go back to the Skaal Village and now wait for 35 days, on the 36 day go back and try again. If that does not help, then leave the island and go back to Skyrim and wait for 35 days and go back to the village on the 36 day. If that does not help sorry then its a major bug.
Only option after that if you still cant mine the Stalhrim is to load a preivious save and try again.