5.3-BETA2 df(1) reports incorrect values for UFS1 FS

On a 4.10 system, my /home reports as:
Filesystem  1K-blocks     Used   Avail Capacity iused   ifree %iused  Mounted on
/dev/ad0s3g  86710002 71758104 8015098    90% 1803049 9038549   17%   /home

When I mount it on a 5.3-BETA2 system, I get:
Filesystem  1K-blocks     Used   Avail Capacity iused    ifree %iused  Mounted on
/dev/ad0s3g  86710002 1929924 77843278     2%   70983 10770615    1%   /home

This is somewhat disconcerting.  As far as I can tell, all the files are
there but 5.3 doesn't realise it.

fsck on 5.3 reports no errors and:
1803048 files, 35879051 used, 42390039 free (1903 frags, 5298517 blocks, 0.0% fragmentation)
fsck on 4.10 reports no errors and:
1803048 files, 35879051 used, 7475950 free (390462 frags, 885656 blocks, 0.9% fragmentation)

Any ideas what is wrong with 5.3?  More critically, is it safe to write
to a UFS1 filesystem with 5.3?

-- 
Peter Jeremy