Sounds like sticky piston(s).
Remove pads and squeeze lever, careful not to pop the piston out!
Clean the sides of the pistons, push them back, then out, repeat cleaning until you see that both pistons are moving at the same rate.
It’s a new caliper.
RS685 caliper do not have the best reputation (or was that the prior generation?) or possible 'flap' issue detailed here:
https://bicycles.stackexchange.com/questions/41573/shimano-rs785-hydraulic-disc-brake-caliper-will-not-reset
@TM started
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Sounds like sticky piston(s).
Remove pads and squeeze lever, careful not to pop the piston out!
Clean the sides of the pistons, push them back, then out, repeat cleaning until you see that both pistons are moving at the same rate.