I’d give my guess that the movement system used in mobile is probably a pretty substantial obstacle to the QA process, owing to all the new area (ginger island, challenge Mines, house upgrades) and shortcuts introduced. QA involving finding where the player can get to places they’re not supposed to, and how well the pathfinding can find it’s way through the playable area. Testing all potential player input and seeing the outcome takes time, as well as fixing whatever does get caught, and altering the playable area or pathfinding to accommodate.
Throw in some COVID shenanigans, and you’ve got the recipe for a lengthy, and probably pretty grueling, project, is what my speculation is. I came to this guess, as the movement system is the single biggest alteration from PC/console to mobile platforms. Outside of this, I can’t think of anything else that would pose a significant hurdle. 1.5 is already running on the Switch, which itself is a mobile device with standard APIs, so I’m fairly certain the holdup is not on the game engine side of things.
Tl,dr: It will be released when it’s released. Doubt we’ll know ahead of time.