I am surprised that nether pants or shirt extended to include forelegs.

Probably because they didn’t extend at all. This particular magic reshape the clothes to fit the shifting body structure, but the law of mass conservation is canon, even for clothing.

Usually, Lukas’ pants fits his bipedal dragon form, save for the tail, and they were at foot level in panel 1. Now, it looks like either his taur form has longer legs, or the pants had to move a few inches of length mass around to make up for the width of Lukas’ potentially wider waist.

or he's still practicing

The clothes haven't got used to Celia yet and are still in the "YOU EXPECT US TO DO WHAT" stage of denial.

Begin the testing limits montage!