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Weird lose of tension in rear derailleur

** Edit: Solved **
When riding yesterday, I experienced a lose of tension in my RD.
If I deliberately put the bike in the 9th gear (or above), whilst still in the lower chainring, the chain would become extremely loose and saggy. spilling over the chainstay.

I appreciate you're not supposed to cross chain. But I've not had this issue before. Example, like when the bike is on a stand and you're cleaning the chain, running it through all gear combinations.

I took the bike to Halfords. They suggested cleaning the drivetrain, shortening the chain and replacing the gear cable.
I've cleaned the parts and shortened the chain. But shortening the chain doesn't seem logical, as I'd want the RD to move towards the rear of the bike to maintain tension.
I haven't replaced the cable (only tried to add tension), because I thought the cable only moved the RD in/out and not towards the rear or front of the bike.

The RD was bought from Halfords last July, so it's still under warranty. I unsure if I've still got the old one, so maybe I could refit it, to check the tension.
The chain isn't worn and neither are the other parts of the drivetrain.

The bike is a 105 R7000 11 speed. 52-34 on the front and 11-40 on the rear. Chain is normally 114 links (standard no. of links in packet), now 112 links.
I've used this setup for many thousands of miles and never had any issues until yesterday.

Thanks for any advice.

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