mechanics question

[Cecil4]

Tough mechanics, not a rule Q. 2 umps.

Runners on 2nd & 3rd, fly to left, R1 tags and starts home. Throw is immediate to F2, R1 retreats, F2 chasing.
PU hustling alongside F2, watching for tag, yells "I have tag", BU nearby.
Just before reaching 3rd, R1 drifts into fair, F2 swivels body blocking PU view; both still moving.
PU yells "BU’s name, yours".
Then R1 dives into 3rd as F2 attempts/applies tag; both now on ground. PU still blocked.
BU close to play, 90 degree to baseline, calls R1 safe as R1 beat tag to base.

Comments ?
USA SB or NFHS