If you enable mod on a save that already has combinators researched, selector doesn't gets unlocked for crafting along with base combinators.

I have been unable to reproduce this issue using the following steps:

1. Disable Selector combinator.
2. Start a new game.
3. Use the editor to research the circuit network.
4. Make sure vanilla combinators are craftable.
5. Save and exit, re-enable Selector combinator, and reload the save.
6. See if a Selector combinator can be crafted (it can).

Are you on the latest version of Factorio (1.1.104)?
Are you seeing this in singleplayer or multiplayer?
Are you using a conversion mod that changes where combinators are in the tech tree?
Would you provide a save file that has this issue?

I also encountered this issue.

I am using the latest version, 1.1.104, singleplayer.

I made a test save using only creative mod for fast research. Here is the save file: https://www.mediafire.com/file/9h3z280edu2v1dg/test.zip/file

The research screen properly shows the selector combinator. I can also use creative mod to give myself the selector combinator. It just doesn't show up in the crafting UI.

Thank you for the additional info; I'm looking in to this.

I'm guessing that the problem is the way Factorio handles technology: all recipes exist to begin with, researching things simply sets the "unlocked" flag on them at the moment of completion. Adding more unlocks to an already completed research does nothing.

As a solution you can do the same thing everyone else is doing: make it a separate cheap research.

Or I guess you can run an on-load script that "manually" enables the selector recipe if combinator technology is already researched. But that's liable to have mod compatibility problems.

Fixed in 0.1.1.