Automatic claim of WMATIC when increasing the stake

Hi there, new to the forum so couldn’t find if this question belonged anywhere else.
I’ve had a small amount in the WETH/DAI farm which was very slowly accumulating some SUSHI and WMATIC.
I then had some more money to stake and decided to put it in the same farm. As I added my liquidity tokens to this stake I noticed the WMATIC ticker for harvesting dropped back to 0 and got into my wallet.

Why is the harvest for WMATIC automatically done when upping your stake in a farm? And why did only the WMATIC come out but the SUSHI’s are still in there?

Thank you for your thoughts.