That's not right. It shouldn't be that thick or stringly, at least not after 10 seconds.
Having not used Tamiya lately, I just stirred up my pretty old bottle of XF-2 flat white and put 10 drops in a pallet.
Excuse the bad pic, but you can see it looks a lot different than what you have.
Here is after about a minute, paint is drying up and balling on the brush tip already, as expected.....
I don't have any Tamiya retarder, but a touch X20A wetted the paint back up.
My hat's off to Bluenote and others who hand paint this stuff. It does sound like Tamiya's own retarder is likely the way to go if you want to hand brush it.
The other idea comes to mind about your bottle of white, maybe the lid didn't get closed well and it is drying out.
But at any rate, back to original question, should white be this much different in consistency then other Tamiya colors? Now that you show the pic, it should not. I half-expected the white in my test to be a little thicker out of hte bottle just because it is white, it was not.
Off-topic, I'd forgotten how much I love the smell of Tamiya Acrylics.