This is a known bug.
Objects tool should stay selected like in ACR.
A fix is on the way hopefully 🤞🏻
For the time being after using Objects and masking is open use Shift + N
You will immediately add a new Objects to the mask.
Use Alt + N to immediately subtract Objects to the mask.
These are the kbsc to add/subtract the last selected tool to the Mask...and yes the modifiers used are the opposite one of what we usually use to add a component to the mask 😵😵
@Rikk Flohr: Photography