I haven't used rockwool myself but I've HEARD of it's ph issues. Next time I'd probably soak the rockwool (and keep checking the ph) until it stabilizes at the ph I want. May take longer than 24 hours.......I know it does when prepping hydroton.

At this point, just keep checking and changing it back which you may have to do several times a day for now. The issue will improve when you get some nutes in there with their ph buffers.....and the problem WILL go away eventually as the medium stabilizes.