Most likely the second coat fouled it up by not being 100% dry. Are you sure a second is allowed by the product? Most do not allow a second coat for this exact reason. Third I do not like Behr but that is me personally and would have used Olympic over the Olympic cleaner. How long did they wait after the cleaner as the deck needs to be good and very dry.

The instructions on the stain call for 2 thin coats. Perhaps there was a trace of moisture when they started. or maybe the coats were too thick, or the temperature dropped too low overnight while initially drying?

But regarless, what can I do now to fix the tackiness? Will it eventually go away?

Staining cedar is always a bad idea. The reason you select and pay all the additional money for cedar is because the wood is filled with oil. Cedar ages naturally so you don't have to apply sealers and stains.

That splintering around the screws should have been a red flag to call back the guy who constructed the deck. See AI comment about the type of screws that are to be used. Also note the holes are to be predrilled. I'm betting your contractor did not do that. Now you have created even bigger problems. You might want to get that finish off the deck, but you also might need to disassemble the floor boards.

Yes, screws are used on cedar decks, but stainless steel screws or specially coated deck screws are essential to prevent corrosive reactions, staining, and unsightly black streaks from the cedar's natural extractives. For cedar, stainless steel (especially 316-grade for coastal areas) is the best choice, but ceramic-coated deck screws are a common alternative, though they may degrade over time. Always pre-drill pilot holes to avoid splitting the soft cedar.

" what can I do now to fix the tackiness? Will it eventually go away? "

Strip it, there are plenty of cases of the decks being tacky years later because they did not strip.