MFGs state...Jack under frame.
CYA lawyerese so that some fool doesn't jack up trailer under axle and bend it then file 'warranty claim' (because mfg. has no control over fools)
Logic....
Trailer is attached to axles by leaf springs and U bolts at spring perch on axles.
Trailer axle weight is supported at that spring perch
Using a jack under axle AT the spring perch will not result in bending the axle.
Tire shop will jack up trailer using 2 floor jacks ...one under each axle at spring perch on one side. R&R both tires then do the other side.
They are not going to crawl up under trailer dragging a floor jack and jack it up in middle of axle when it would be damaged.
Too much of a hassle, too much time involved, too big of an effort.
All of which a tire shop is NOT interested in doing
Relax...