Yes, it is!
Yes, it is a normal occurence.
There is so much demand on the green system during extension that the requirements for PTU start of operation are met. Green ENG 1 pump does not have enough capacity to hold the Green system pressure above 2500 psi during gear extension/retraction, so briefly, the PTU will kick in. Should the yellow system not operate, the gear cycle will just take a bit longer to complete.