The fdisk -l shows that you only have one lvm partition (type 8e) sda3. sda4 is a standard linux partition. Normally a pvcreate won't let you create on a non lvm type partition, but as we're dealing with a luks mapping that may explain why pvcreate managed to create a pv on it.
Worryingly, the pvs command shows that its finding a lvm physical volume, on sda3 which it shouldn't as that is the encrypted device and it should only find it on the /dev/mapper device associated with it when you open it. Your psize/pfree show it as being empty and its not allocated to a volume group. Here's mine for comparison purposes.
PV VG Fmt Attr PSize PFree
/dev/mapper/pvhda2 sysvg lvm2 a- 18.91G 6.91G
/dev/mapper/pvhdb1 datavg lvm2 a- 149.05G 97.05G
stranger still is that once unlocked, the sda4/dm-0 device doesn't show any information either.
Are these commands being run from your running debian system or a live/recovery cd?
If you're running from a live cd, try a
and see if that changes the output of pvs.
If they're from your debian system I'd be interested to see the output of commands.
My gut feeling is that somehow during the debian install a new physical volume has been created over the sda3 partition wiping out the existing luks metadata. The unopened sda3 would have just looked like random data to the installer so there's no way it could have known it was already in use. Your sda4 appears to have a pv header on it, but at this stage its not clear whether its actually being used as such.