I am having this exact issue after pushing in pistons when replacing the brake pads (as manual instructs to do).

"Use a flat-shaped tool to push the
pistons straight back in as far as they will
go, while being careful not to twist the
pistons.
Do not push the pistons with a sharp
tool.
The pistons may be damaged."

The way it manifested is that some time after changing pads, the lever went soft, and I noticed a drop of fluid under the cover. Lever would get hard after some pumping, but would go soft again.
Today I bled the brakes, and the lever was initially nice and firm, but then went soft again. I checked again under the hood and found this leak on the side of the lever, and also this bubbling when squeezing brake.
It looks like this video I found:
And is apparently a common failure. They should either build in some failsafe, or warn people to open bleed valve when pushing piston.
I bought my bike new less than 2 years ago, so hopefully Shimano warranty covers it, will be contacting them directly tomorrow.
Anyone deal with them directly on OEM parts pre-installed on bike? I don't want to go to the bike store due to COVID.