So Export As is broken but you have two options with CMYK there (there should be none):
1. Convert to RGB ON.
2. Convert to RGB OFF.
If you select Convert to RGB, you end up with an untagged document in sRGB so you'll have to use the Assign Profile command (or just don't even use this buggy means to produce a PNG).
If the check box is off, it converts to RGB based on the RGB Working Space which is even worse.
Bottom line: if you have CMYK, stay far away from Export As.
Author “Color Management for Photographers" & "Photoshop CC Color Management/pluralsight"
The client wants print file, so is it okey of I send rgb format? I sent png file in CMYK format before :(( should I send it again amd tell what happend?