Has the Biden administration provided any justification for the timing of the announcement (i.e., in lame duck period, and right after the Scholz-Putin call)?
First, the White House was reluctant to comment, with no official announcement of the kind you imply for days.
The Biden administration hasn't publicly confirmed the ATACMS policy change. State Department spokesperson Matthew Miller told journalists at a Monday briefing he would not confirm or deny approval for ATACMS use inside Russia, but said the U.S. response to Russian and North Korean military cooperation in the war "would be firm."
Likewise:
In a Monday [Nov 18] morning briefing in Rio de Janeiro, U.S. Deputy National Security Adviser Jonathan Finer evaded questions from reporters about Biden's [ATACMS] authorization.
"I'm not confirming any decisions that have or have not been made about U.S. assistance when it comes to these operational questions," Finer said. "I will say, with regard to the comments that came out of Russia, the fire was lit by Russia's invasion of Ukraine."
Eventually,
The White House for the first time on Monday [Nov 25] publicly acknowledged easing restrictions over Ukraine's use US-supplied longer-range missiles, allowing Kyiv's forces to strike targets deeper inside Russia.
National Security Council spokesperson John Kirby said Washington "did change the guidance" over Army Tactical Missile System (ATACMS) missiles, "and we gave them guidance that they could use them to strike these particular types of targets."
"Right now, they are able to use ATACMS to defend themselves in an immediate-need basis. And right now, understandably, that's taken place in around Kursk, in the Kursk Oblast," he told reporters, referring to the Russian territory Ukrainian forces have been operating in since August.
"I'd let the Ukrainians speak to their use of ATACMS, and their targeting procedures, and what they're using them for, and how well they're doing," he added.
There's the full WH press conference of Nov 25 transcript available, but it doesn't say much more about this.
There's not been a lot of these strikes, despite all the press. Perhaps two strikes by Ukraine with ATACMS in Bryansk and on a airfield in Kursk, to date. Commentators who were saying that their impact would be limited because of limited supply were probably correct.
BTW, I don't see why the WH would have to be terribly concerned about the Scholz-Putin call, of all things. Despite some (ahem, CNN) headlines that the call was "ending Western isolation" for Putin (headline that they've changed in the meantime, by the way), little of that sort came out from the summary of the call. And Scholz has now visited Ukraine again. The call appears to have been more of an attempt to placate domestic opposition (ahead of the elections) that Germany was not doing enough diplomacy. Anyhow, no WH press corps journalist seems to have made the connection with ATACMS to ask something like that. However, based on Dec 2 press conference with Kirby, the general gist of what the WH is doing is (in their opinion):
White House national security spokesman John Kirby emphasized Monday [Dec 2] that the Biden administration’s job is to put Ukraine in the best position of strength to give Zelenskyy leverage when negotiations begin. He underlined that it’s Zelenskyy “who gets to decide if and when he’s ready to negotiate, and he gets to decide what if anything he’s willing to negotiate.”