gddrescue --fill-mode=+ with a map file of 0x00000000 0x608336000 + has left the partition table intact?

I first ran this command:

ddrescue --force /dev/sda /dev/null HGST_RMA.map

I just wanted a map file for blanking (therefore /dev/null). And this is what my .map file looks like before I interrupted the run towards the end (Ctrl+C'd it):

Then I decided to blank the disk (/dev/zero):

$ sudo ddrescue --force --fill-mode=+ /dev/zero /dev/sda HGST_RMA.map
GNU ddrescue 1.21
Press Ctrl-C to interrupt
Initial status (read from mapfile)
filled size:     266742 MB,  filled areas:         43
remaining size:  233363 MB,  remaining areas:     151
Current status
filled size:  500106 MB,  filled areas:    194,  current rate:  55115 kB/s
remain size:        0 B,  remain areas:      0,  average rate:  71200 kB/s
current pos:  500107 MB,  run time:     54m 37s
Finished         

Then I ran:

$ fdisk -l /dev/sda
Disk /dev/sda: 465.8 GiB, 500107862016 bytes, 976773168 sectors
Units: sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 4096 bytes
I/O size (minimum/optimal): 4096 bytes / 4096 bytes
Disklabel type: dos
Disk identifier: 0xbf7dbd9f

Device     Boot    Start       End   Sectors   Size Id Type
/dev/sda1           2048  62916607  62914560    30G 83 Linux
/dev/sda2       62916608  67110911   4194304     2G 82 Linux swap / Solaris
/dev/sda3       67110912 976773167 909662256 433.8G 83 Linux

This is what the initial bit of my map file looks like:

# Mapfile. Created by GNU ddrescue version 1.21
# Command line: ddrescue --force --fill-mode=+ /dev/zero /dev/sda HGST_RMA.map
# Start time:   2017-12-28 17:37:14
# Current time: 2017-12-28 18:31:57
# Finished
# current_pos  current_status
0x7470BFD000     +
#      pos        size  status
0x00000000  0x608336000  +
0x608336000  0x00001000  -
0x608337000  0x380B8DD000  +
0x3E13C14000  0x00001000  -
0x3E13C15000  0x00224000  +

By my own estimate the partition table should have been wiped clean because of the

0x00000000  0x608336000  +

Why hasn't that happened and what has been blanked?

What has been blanked?

The clue is here:

$ sudo ddrescue --force --fill-mode=+ /dev/zero /dev/sda HGST_RMA.map
…
Initial status (read from mapfile)
filled size:     266742 MB,  filled areas:         43
remaining size:  233363 MB,  remaining areas:     151
…

For some reason ddrescue decided it should continue from the middle of the disk, as if the first half had already been filled. This way the partition table was left intact.

Why […]?

Frankly I cannot tell. My tests with ddrescue 1.19 indicate this happens when the current_status inside the mapfile is F meaning "filling specified blocks". This is the case after you interrupt filling operation. Continuing seems sane, I would expect this.

But according to your ddrescueview screenshot, the current_status was *, "trimming non-trimmed blocks". With this status I cannot replicate the issue, my ddrescue fills all healthy blocks as it should. Again, this is sane and expected.

Either you hit a bug or you had altered the file(s) (and neglected to mention it here) in a way that mislead ddrescue. E.g. if you accidentally run this:

sudo ddrescue --force --fill-mode=+ /dev/zero /dev/null HGST_RMA.map

and realized your mistake ("Wait! Why is it writing so fast?"), interrupted halfway with Ctrl+C, then your mapfile would contain F as current_status. Next you run the proper command and it "continues" from the middle of /dev/sda.